Y'ALL HELP (multi class shenanigans)

I'm currently having a dispute with another player on muti classing and how it should work, because with the way it normally works. One character would be a way higher level than another (making them weeker overall) and it would be a balancing nightmare for our dm. My proposal was if you gain (insert amount of XP) you could split it up into each class or put 75%-100% in one class or another. I don't know what to do but I really want to play a multi class. Thank you for coming to my Ted talk (please help)