Permanent house rule in my game, 5th level casters (aka when martials get 2nd attack) can maintain concentration on two spells. One concentration check is made, on a failure both spells are lost.In my next short campaign I'll experiment with lifting this cap on concentration to 1+1/2 proficiency bonus, i.e. casters of level 1 to 4 can concentrate on up to 2 spells simultaneously, casters of level 5-8 on 3, etc.
Used it in 3 campaigns so far, never seen a single problem with it.