Dual Focus

Countless hours have been spent training your mind to maintain focus on concurrent incantations, taxing as the process may be.

• If you attempt to cast a spell that requires concentration while already concentrating on an existing spell, you can maintain concentration on both spells simultaneously. You must spend a standard action each of your turns on maintaining this concentration, or lose concentration for both spells. When you take this action, you must make a Constitution saving throw* (DC equals the added level of both spells to a minimum of 10 + the number of turns you’ve been concentrating on two spells). On a failure, you lose concentration for both spells. You can drop concentration on one of your spells during your turn as a free action to avoid this saving throw.
• Any time you would be forced to make a Constitution saving throw to maintain concentration due to taking damage, you roll with disadvantage, and instead of the DC equaling half the damage taken the DC equals the full damage taken to a minimum of 10. On a failure, you lose concentration on both spells. You can willingly drop concentration on one of the spells to take the concentration check normally.

*This saving throw counts as a concentration check.

- Matthew Mercer, Tal'Dorei Campaign Setting, Pg. 108. (My alternate take)