Voadam
Legend
In AD&D it was closer to doubling xp cost each level so if you were multiclassed you would usually be around only one level behind where you would be otherwise, not half the levels. Once you got past the first level 2 advancement for non multiclassed ones everyone in practice was advancing a new level at roughly the same time.This is how multi-classing used to work. In 1e (where the Gish concept comes from), you earned experience in both (or all three) of your classes at the same time. But this worked because experience kept your progression slower, since the only way to level was through earning experience points, and thus you levelled at approximately half (or a third) the speed of single-classed characters (I'm aware that due to the way experience points scaled, there is some variability). There were also level limits imposed on most classes available to multi-class, since it was an option for non-humans only.
How do you make that work in 5e, where experience points are not used at many tables, and where players expect to level at the same rate? Obviously, if your fighter/wizard can be level 5 in both classes in the same amount of play it takes my fighter to be level 5, that's unfair and unbalanced. So how do you fix it?
One obvious solution would be to simply do 5e multi-classing, and switch classes every time you level (e.g. go fighter 1->wizard 1->fighter 2->wizard 2->fighter 3 and so on). But I get the sense that this would be unsatisfying for most players who post about playing a Gish, even though it would produce pretty much the same mechanical result as 1e multi-classing.
Or I suppose if doing milestone levelling, the Gish could just level half as often, but level both classes at the same time.
Also you kind of describe it backwards for multiclassing and level limits. Demihumans who can multiclass have level limits on most every class whether they multiclass or not. Multiclassing is not the reason for the level limits being demihuman is.
In 3.5 the Unearthed Arcana Gestalt option to advance in two levels at the same time was fairly similar to AD&D multiclassing. It was mostly setup as a campaign option to make PCs more broadly powerful. Action Economy was a big limiting factor to the power increase.
To do a 5e one and be balanced against single classed characters I would suggest something like a 3.5 level adjustment, so you could be a caster with both wizard and cleric spells and cleric armor and HD, but you would be a level lower than a straight cleric and have lower hp and lower top spells every other level and later ASIs and proficiency bonus advancements. A potentially interesting tradeoff or power options.