The problem with the tabletop spell + cantrip rule is that it combined with random initiative order severely prohibits spell combinations. Full spells + cantrips have too little combinations to even work with. As for levelled spells, even if you have 2 party members with synergistic spells like Web and Fireball, they can be easily thrown around the order by the initiative roll the way that by the time the Fireball wielder gets his turn, target(s) might get their turn earlier and just break and run out of the Web wasting a spell combo just because of bad RNG. Moreover, Web wielder could just get his turn after the Fireball wielder to make things even worse.
The correct answer to this could be allowing PC's to delay their turn to go in the same initiative pocket with another PC.
I think being able to cast two spells per turn is a balance problem. Misty Step + Thunderwave before the target gets any chance to react is already too much. Enemies need to be able to do the same and then it just gets ridiculous.
