Someone in this thread performed the Rite of Ashk'Ente.
The intent was that a magus could not cast a fast cast and a normal spell in the same round.
However, I take the point that everyone would want to go last, so I suggest treating each character's round as starting immediately after their normal initiative point, when it matters. Thus, if you've cast a normal spell, and your turn has passed, you can cast a fast-cast spell, but you can't then cast a normal spell in the next round. Essentially, you start casting your next normal spell as soon as the last one goes off, but you can abandon it to fast cast.