Maybe it's 2e that made this official, or maybe it never was, but we've house-ruled it this way since forever in 1e: your classes advance independent of each other. You determine the ratio of xp you're going to dump into each class - say, a F-MU that's 75% Fighter, 25% MU - and each class just bumps when it bumps.
So you start as a 1-1 with 0 xp on each side. During your first few adventures you earn 4000 xp, which puts 3000 on your F side and 1000 on your MU side - you're now F-2/MU-1. Get another 4000 and you'll have 6000 on your F side (so now 3rd level) and 2000 on your MU side (still 1st level but 2nd is getting close). Do this again, so now you've got 9000 on your F side and 3000 on your MU side, and you're a F-4/MU-2....and so it goes.
See how this works?
There's nuances - we allow the ratio to be changed between adventures, for example, to reflect character development. In the above example, the player-as-PC might now decide to focus more on magic use, and flip the xp ratio for the next adventure to 75% MU/25% F. You always have to put at least 10% into a class, thus 90-10 is the most extreme ratio we allow.
It certainly requires a little more arithmetic on the player side, but I see this as their problem not mine.