it's the other way around Tom , host prog first.. then when you start the slave one it checks that state, sets it accordingly (I had to look up to make double sure, hehe)

*BUT* normally with reason, it's a case of just doing insert rewire device in sonar.

the only things I'd check are 

1)both soundcard setups are correct / allow sharing
2)the rewire lib is up to date
3)both things are either 32 or 64 bit

what do m-audio say about sibelius ? (I gave up on avid tech support = give wrong info back / won't buy their products now)