It does not seem possible with NWC 1.30f to write duplets (two notes in the timing of 3). In my case two sets of duplet notes in a time signature of 6/8.
You can do duplets (2 over 3) by using dotted note values. E.g., over 3 8th notes you can do 2 dotted eighths.
OK, this won't solve the quintuplet problem, but it helps.
- seb