Yea, I know it's been discussed around here before, I think it's a pretty good idea, personally.The big sticking point is that some classes have much better subclasses then others and they synergize better. But I don't think the difference is so large that you couldn't manage it for your own game with a little bit of targeted magic item assistance to the less strong player.
Fun combinations off the top of my head:
Lore/Valor bard (Armor and Weapon options, Extra Attack plus extra spells to make you a better gish).
Arcana/Nature cleric (Heavy armor, start with the Shillelagh/Booming Blade combo right off the bat, get Divine Strike and Potent Cantrip at 8).
Battlemaster/Eldritch Knight Fighter. A fighter with potent short rest and long rest options. This would be pretty compelling.
Gloomstalker/Beast Master Ranger (assuming class variant UA fixes for the Beast Master).
Moon/pretty much anything else Druid. Better wild shape + better spellcasting.
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