What are the impacts of permitting casters to concentrate on 2 spells?

Question

A common lament at tables I play at from players is often the inherent limitation on being able to maintain concentration on only one spell at a time. This doesn't just come from casters, but also other players wanting to be able to enjoy multiple buff spells at the same time. I find this concern comes up most often during higher-level play where characters generally find themselves having more spells than they are ever likely to use in a day.

In addition, I have found in my own experience playing a sorcerer with a limited spell list that concentration spells are less attractive due to feeling like once I cast one, I can only use the other spells on my list that don't require concentration. This turns an already small list of available spells into an even smaller list.

For the sake of defining parameters, our tables often consist of 4 players creating a balanced group of 2 martial types and 2 caster types. The actual classes chosen are more or less irrelevant to the issue (I think).

The proposed class feature would be added to 11th level for the following caster classes (Bard, Cleric, Druid, Sorcerer, Warlock, and Wizard):

Improved Concentration

Beginning at 11th level, you may maintain concentration on up to two spells at the same time. Neither spell may be higher than 5th level. If you are required to make a Constitution saving throw to maintain concentration, you must roll separately for each spell you are maintaining concentration on; failure causes you to lose concentration on that spell.

You may also maintain concentration on one spell while readying a second spell. You may not ready a spell while already concentrating on two spells without losing concentration on an existing spell of your choice.

I anticipate that this will make full casters more powerful than they already are, but will also serve to improve the overall power of the party via additional buffing. The increase in power doesn't concern me, but what I am concerned with is whether this opens the door to potentially game-breaking combinations that didn't exist prior to this rule.

The thought process on this was as follows:

• The level requirement is set to more or less make this the domain of dedicated full casters and not the sort of feature that can be acquired by a small level dip. Furthermore, the level selection ensures that levels where spell levels 1-5 see the most action are not diminished by the feature.
• The spell level requirement is set to prevent using 2 extremely powerful concentration spells simultaneously (e.g. no maze AND Otto's irresistible dance at the same time). The in-game threshold on extremely powerful appears to be anything above spell level 5.
• This will open up the known spells list a bit more for spontaneous casters.
• This will generally just make high level play a bit more rocket taggy in nature.

Assuming play at 11th level and higher, would this proposed house rule break the game in a way that couldn't have been done by 2 characters working together in standard RAW?

A good answer would stem from actual experience associated with using a rule like this and what the implications of that were. Recommendations stemming from that experience on how to nerf or buff it to better control the power (limit the spell levels further, increase class restrictions, limit by school, etc.) should also be included.

Answer 1

The Dungeon Master's Guide strongly recommends against this.

Dungeon Master's Guide, p.263:

Beware of adding anything to your game that allows a character to concentrate on more than one effect at a time, use more than one reaction or bonus action per round, or attune to more than three magic items at a time. Rules and game elements that override the rules for concentration, reactions, bonus actions, and magic item attunement can seriously unbalance or overcomplicate your game.

Here's why:

In earlier editions of the game, which had no one-spell concentration limit, characters could load up on buff spells before a fight or summon multiple creatures at once. Preparation rounds would give a significant advantage. It rewarded players who cast all their daily spells in one combat, leading to the "five minute adventuring day", where the party fully rests after every encounter.

It also overcomplicated the game because players often had to track multiple ongoing spell effects. It was too easy to forget your effects, leading to annoying situations where a player demands a do-over on three rounds of combat he forgot he was invisible, or a character who dies only to pop back to life when they remember a defensive spell that should have applied. I can't count the number of times someone in my D&D 3rd edition game forgot to apply an effect.

D&D 4th edition powers (i.e. attacks, spells and the like) often caused status effects, which were likewise easy to forget and a real hassle to track. D&D 5e's decision to limit each spellcaster to a single ongoing effect considerably reduces the number of possible "buffs" and "debuffs" on the battlefield at any one time. The DM spends less time adjudicating and tracking ongoing effects, which speeds gameplay. In my personal experience, combat could be very slow in D&D 4e and high-level D&D 3e, so the one-effect concentration limit is of significant importance.

Answer 2

Few game breaking combos: but constant game breaking imbalance

First of all, you said that

I anticipate that this will make full casters more powerful than they already are... The increase in power doesn't concern me

I want to stress that this proposed class feature will double the effectiveness of most full casters, especially ones which rely on battlefield control or buff spells, but also ones which rely on pure damage. It would be similar to allowing all martial classes to have two actions every turn after 11th level. Unless you are doubling the damage output of all the other classes as well, you seriously run the risk of making players running martial characters feel practically irrelevant.

Since you specifically asked for effects that "couldn't have been done by 2 characters working together in standard RAW," consider that your proposed casters could stack two concentration spells with a range of "Self" on themselves, something which is normally only doable with the assistance of a Spell Glyph (which is difficult to use in most situations, as its location is fixed). This would make certain martial dip-full casters extraordinarily powerful: for example, a Warlock/Vengeance Paladin could combine Hex and Hunter's Mark on themselves and add an additional 2d6 to all their weapon attacks. Similarly, a Bard could combine Hunter's Mark and Swift Quiver (which doesn't have a range of "Self," but only buffs the caster).

These self buff combinations would be the main thing that a single caster with this feature could do that two dedicated casters couldn't do normally. There could be some odd combinations of spells (such as Scrying being used while you ready a touch spell to be delivered via a familiar in the area Scrying is observing), but most of them could be attained by existing class features (e.g. the Warlock invocation Voice of the Chain Master). But I urge you once again to reconsider. When considering game breaking imbalance, "two characters working together could do this" is not a reasonable metric for what a single character should be able to do alone.

Answer 3

It would be unbalanced; martial classes would be even weaker relative to casters

Even in the current edition as-is, mages could be overpowered above fighters - for example, if your GM regularly uses only one encounter per long rest. In such cases, mages can spend all their spells at once (dealing significantly more damage, and taking less).

Now back to your rule - If I played a fighter in your group, I would feel useless in comparison to caster characters. It would be less fun for me and I would probably quit playing the fighter class.

On the other hand - some boss mage could have this feature. That could be fun to fight with him :)