Fanaelialae
Legend
You sir have intrigued me never played AD&D im assuming there was a table that governed the level gap somewhere but this sounds interesting.
Yeah, sorry, I was assuming anyone who read that would be familiar with AD&D.
Basically, the way that multiclassing worked in AD&D was that you picked two or three classes (though the combinations were limited by your race) and split XP between those classes evenly. The rest of the rules above are largely equivalent to the AD&D rules.
The above "level limits" are simply the result of doubling/tripling the amount of XP needed to gain a level in 5e. A two-classed character is 15th level when his single classed friend is 20th because it normally takes 355,000 xp to reach 20 level. If you split that evenly between two classes, you get 175,500 xp which is only enough for level 15.
In AD&D you tracked the xp for each class separately because each class required a different amount of xp to level (IIRC, a thief needed 1,250 xp to reach level 2, whereas a wizard needed 2,500 xp for the same). In 5e you could simplify this by simply creating xp tables that are double/triple the normal xp table (that way you save the players the trouble of having to divide xp constantly).