Currently looking at tanking options, I understand that you cannot add the AC of 2 shields together. However, it is possible to wield 2 shields. Does this mean you can benefit two different magical properties?
So would you gain both:
Player's Handbook, p.144:
You can benefit from only one shield at a time.
It doesn't say you only gain the AC bonus from one shield, it says you cannot benefit from more than one shield. If you attach a second shield, only one of your shields has effect (unless a more specific rule elsewhere overrides that).
Note that it doesn't say which of your two shields has effect, if you've attached two. You could, arguably, swap between them ambidextrously as benefits you, but there's no particular rule covering this rare case.
Dungeon Master's Guide, p.141:
Multiple Items of the Same Kind
Use common sense to determine whether more than one of a given kind of magic item can be worn. A character can't normally wear more than one pair of footwear, one pair of gloves or gauntlets, one pair of bracers, one suit of armor, one item of headwear, and one cloak.
This doesn't explicitly describe shields, so it doesn't override the general rule that you can only benefit from one shield at a time. The fact that it doesn't say "you can only use one shield" doesn't implicitly mean that you can use two shields, because the general rule for shields is that there can be only one.
However, it does advise the DM to "use common sense", which means that it's up to the DM whether or not you can actually wear and benefit from two shields.
Your reasoning seems sound to me. They aren't duplicates of the same magic item, you should be able to benefit from both of their effects (the +3 from one and the magical effects from the other).
From DMG pg. 252 (added in errata):
Combining Game Effects
Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap. [...] Game features include spells, class features, feats, racial traits, monster abilities, and magic items.
We know that multiple sources of the same item don't stack, so as you've pointed out, you can only get the AC bonus from one of them.
From PHB pg. 144 (or here in the basic rules), under "Armor and Shields":
You can benefit from only one shield at a time.
Regarding being able to use two magic shields at the same time, see DMG pg. 141 (or here in the basic rules):
Multiple Items of the Same Kind
Use common sense to determine whether more than one kind of a given magic item can be worn. A character can't normally wear more than one pair of footwear, one pair of gloves or gauntlets, one pair of bracers, one suit of armor, one item of headwear, and one cloak. You can make exceptions; a character might be able to wear a circlet under a helmet, for example, or be able to layer two cloaks.
So you would need to actually wield both shields, thus leaving no hands free for weapons. Of course, if you have the War Caster feat, you can still cast (some) spells (so long as they don't have material components, or you are of a class that can use a shield as a focus), or you can use (one of) them as improvised weapons for 1d4 + STR damage.
The phrase of not being able to receive benefits of two shields at the same time was referring to normal shields in regards to AC.
One could use two shields and in actual history has been done by soldiers protecting others. One can use two rings one on each hand. One can use two shields one in each hand and receive the magical benefits but not AC is what its telling me.
Also, if using two shields your hands are occupied and perhaps not viewed as a threat or "turtling" so not attacked anyways.
Edit for better explanation per Davids request: You can not benefit from two rings of protection same effect, does not stack, two shield bonuses AC same thing don't stack. Ring protection and ring of fire resistance, both work not stacking same effect. Shield of fire resistance and spell guard shield would both work but AC bonus from only one. Replace shield of fire resistance with shield +3 same thing gets only best AC but also other shield. No historical evidence of two shield users for attacking purposes though there are shields made for that, spikes and bladed ones in the Royal Armoury. Historical evidence of "shield bearers" used only shields defensively to protect their lords/nobles 11th century manuscript in British Library. Also used to protect siege equipment operators, like rams, sappers, archers from other archers.
This is a slippery slope, b/c if you let people gain the benefits of multiple magic items when usually only 1 is gained, then what's to stop someone from getting like ... 4 magical shields... strap one to their back, one to their front, and have one in each arm.. then tell the GM that all the magical effects the shields give are working at the same time.
It would be madness, I say.. MADNESS!
But, honestly, it should be a GM judgement call.
The GM may decide that the Spell Guard Shield benefits you all the time as long as it's on your person. Some magic items work like that, where as long as you have it, you benefit from it. You only gain the AC bonuses of it if you use it actively. But, if you had the Spell Guard shield strapped over your back then the GM may say that it imparts the saving throw bonus (maybe it creates a magial aura around you, or maybe the GM would say it always creates a spell guard aura in front of itself, so having it on your back means your back gets extra saving throw protection.. which could be simply "spell guard saving throw bonus / 2", because unless your GM wants to get anal retentive about which direction you're being attacked from (which may be the case in situations like backstabs or a magician sneak-attacking you with magic from behind) a good rule of thumb is to just abstract things.. so assuming the spell guard is PASSIVELY being used strapped to you back, you'd only gain half of it's bonus via it protecting your back.
As far as two shields, though.. I don't know why there would be rules against using 2 shields for extra protection. In history, some combatants would use 2 shields to protect themselves from both sides.
The point of AC is that its an abstraction of both damage avoidance and soaking. You may dodge a blow, or your shield may block it, or it gets by and your armor blocks it.. all of that would be covered by AC in that "the blow didn't touch you". (This is why traditionally in D&D, you have dexterity added in for an AC bonus.. b/c tough armor makes it hard to penetrate, but good reflexes make you hard to hit in the first place).
you can argue that having 2 shields means you can double-up your active protection by having two moveable blockades around you.
But, you can get into arguments over "combat awareness". A person can only focus on so many things at once. So, they notice something coming at them from one direction, they block with a shield. They notice something from the other direction, they block with the other shield. But, while they're raising shields to block everything.. it may lower their combat awareness as their vision gets blocked.. so they dont' notice someone with a spear jabbing it under the spears into their leg.
So, there's arguments for both gaining a bonus and for not getting a bonus since having to mess with both shields means you lose some vision, combat awareness, etc.
I know that's not what you were asking for, though.. you simply wanted to know if you'd gain the special benefits of the spell guard shield.
Honestly... you can argue this from another point...
"What if I strap the spell guard shield over the + 3 shield?"
You would probably get a penalty for it being awkward and unweildly, but if both are of fine craftsmanship and light-weight...
But, "layering" stuff has always been a point of contention in D&D. Back when they were differentiating shields as bucklers, shields and tower shields, you'd always have somebody go "what if I nail a few buckler+5's to my tower shield +3? Wouldn't that provide extra protection?"
The answer in invariable ... "it depends on what the GM allows".
But, what most GM's say is that the +X's and magic of an object are inherent to it as-is.. not if it's being layered or used as a part of some greater contraption.
EG: a buckler +5 would be very fine crafstmanship, and perhaps give a fighter heightened combat senses (whatever excuse to justify the +5). All of that is lost if you nail it to a tower shield, b/c you're no longer actively using it.
But, as I said earlier, if a power in an object forms a sort of "aura" around the player.. then just having the object on you may be enough. This isn't always the case, as rings, necklaces, etc have to be worn to gain their power. But, if you have an intelligent sword, it can telepathy with you even if it's sheathed.
So, unless there's specific descriptions about the object to help you and the GM come to a decision, it's really up to the GM.
And, a good GM will decide to go with what's best for power-balance of the game. Because the moment they start letting someone get away with something like using 2 shields, but only gaining the power perk of one of them while +X's from another.. there's going to be other players going "gee, how can I abuse this new loophole".. and if you have any munchkin players, they're going to start huge arguments to try to throw off the power-balance of the game in their favor.
