It would be good if there was setting in Modul8 for setting for output to both syphon and 2nd monitor. I can see a couple of situations where would be handy. Namely being able to record some/all of a set outside of Modul8. As the internal recorder seems to suck all the cpu away making it mostly useless in a live environment. Also the possibly streaming your set via a 3rd party program to uStream/Just.in.
Even if you just added a 3rd output menu option (Full Screen, Syphon, Full Screen + Syphon) would work.