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.
That is why I suspect Larian did tweak the 5e spell rule for the first place - not because they didn't understand it, but more to promote creative thinking and actually allow many combos to happen without dependance on RNG as you execute them yourself. This has a downside that 2 spells / turn often is (close to) OP, but it can be solved via just limiting access to such a possibility. In Sorcerer's case, there are many ways to do it - from increased Sorcery Point cost for Quicken Spell if it is a levelled spell (as they already did, but in general) to manual limits like X times per short / long rest.
