There's no such thing as "between" turns. You're either on Turn 1 or Turn 2. They might do something where you prepare to cast on Turn 1, and so do nothing else that turn, and can then cast on Turn 2, but not prepare to cast on Turn 1, cast "between" turns and can then do another action on Turn 2 (that would defeat the point of having some spells take a turn to prepare).
I agree, if you want to enforce the turn system strictly, but in a lot of games a turn involves taking just one action whereas in D:OS you can take multiple actions each turn, which doesn't make sense in regards to time lapse. If you do not like the idea for some reason then that is what is important.
