Thinking about hybrid multiclassing in 1D&D...
For spellcasters, it's pretty easy. Each half of the hybrid class gives 50% of the spell slots you'd get for the full class. (It's a little challenging to figure out how to deal with the odd numbers, but there are various ways one could tackle that.) Generally, any time you've got something that's usable X times per day (or per short rest), you can just cut X in half.
For martials, it's tougher; all non-rogue martials rely primarily on Extra Attack, which is not so easily split up. A solution would be to say that you can't use it if you used it on your last turn. (If you have Extra Attack from both halves, obviously, they add up to regular Extra Attack.)
Rogues could do the same thing with Cunning Action, and then reduce their Sneak Attack bonus damage to 1d4 per three levels. (Or 1d6 per four levels, but 1d4 per three offers a smoother progression for the same average damage.)