only multiclass that I like would be even split, but that is really awful in 5e, maybe if levels for Extra attack stack, martials could got away with it as in 3.5e with base attack bonus,
we played with version of multiclass where you are even split between 2 classes but on levels 5,8,11,14,17 and 20 you get 2 class levels instead of one, you still get only one increase of HPs(alternating between lower and higher HP gain), HDs and proficiency bonus.
I.E: fighter wizard at 20th level would have features of 13th level fighter and 13th level wizard, but still be 20th level total with +6 proficiency bonus and HPs of 10th level fighter and 10th level wizard.
with one extra limit, cannot have 2 full spellcasters multiclass.