This question is prompted by both Sage Advice and this similar Pathfinder question. Let's use the scenario from Sage Advice:
Can you cast a reaction spell on your turn?
You sure can!
Here’s a common way for it to happen: Cornelius the wizard is casting fireball on his turn, and his foe casts counterspell on him. Cornelius also has counterspell prepared, so he uses his reaction to cast it and break his foe’s counterspell before it can stop fireball.
In this case, it seems as though Cornelius would be casting 2 spells at the same time – Fireball; and (before Fireball is complete) Counterspell.
The order of events alluded to by Sage Advice would be:
- Cornelius begins casting Fireball
- Cornelius' opponent begins casting Counterspell
- Cornelius casts Counterspell, preventing his opponent's spell
- Cornelius finishes casting Fireball
- There is much fire
So far as I've been able to tell, it seems that the example given in Sage Advice would result in Cornelius essentially casting Fireball and Counterspell simultaneously.
By extension of the same mechanic, a caster could cast Shield in response to an Opportunity Attack by an opponent with the Mage Slayer feat. A Warlock could use Hellish Rebuke in the same circumstance.
This doesn't seem correct, at first glance, as it means that a caster would be able to do multiple complex somatic components, or to incant arbitrarily many verbal components simultaneously.
This seems strange enough that it may be that the Sage Advice column has overlooked a rule preventing this. Alternatively, they may have just skipped an implicit "but you give up the fireball by doing this, of course".
In short, can a character effectively and successfully cast two spells at the same time?
Best Answer
Yes, you can cast multiple spells at the same time and have both take effect
There are no rules against this
The rules are fairly clear about what is and is not allowed with regards to when and how spells can be cast. The restrictions on spellcasting timing come down to two major rule categories:
As long as you have the action available to cast the spell with all the appropriate rules that all spells must follow then you can cast it. There is no rule that restricts casting with respect to ongoing casting and certainly nothing that implies that it is in any way cancelled.
Sage Advice Compendium explicitly allows it and there appears to be no mistake
You quoted it and I understand being sceptical, but there is no reason to think that this is a mistake. It contradicts no rules and it is an official rules clarification document.
If counterspell did not work this way, there would be no way to counter a counterspell to one of your own spells. And the whole point of this Sage Advice was to clarify that counterspell is intended to allow this.
However, that is not the only example of casting spells at the same time.
Spells with casting times longer than 1 action/reaction is another place where this could happen
Take this case for example:
In this case, you do actually kind of cast two spells at the same time. And this should work with any type of spell cast as a reaction during the casting of a spell with a duration greater than 1 action or reaction. As long as the caster maintains concentration and uses their action every turn to continue casting the long spell, they can do whatever they want.
Note that this will not work with bonus action spells because the restriction on those means that only 1 action cantrips can be cast the same turn.