Main issue with your idea is that fights are balanced around lasting 3-4 rounds. So a spell that takes so long to cast will be useless. In DnD there are a hole bunch of spells that take 10 or sometimes 100 rounds. But you most likely won't use them in combat (except in some story bits).
Combining spells from several casters is problematic because of initiative order. And in most cases casting 2 spells will be more effective then casting 1 with a stronger effect (for the same reason True strike is a trap chose)
