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.