Ok...I had a Powertwin but not the Pro version. It would only fit the non-pro seat as far as I am aware. But...you do not necessarily need the car seat clips to clip the car seat on. The car seat (ie pro or non-pro)does actually nestle into the buggy when the the back seat is reclined, it doesn't move about once its in. I actually preferred using it this way as it meant you could still use the rear sun hood and completely cover up baby if sunny or drizzling. (the car seat clips click into the hood slot, if you are with me).
Does that make sense?
Good idea getting a separate Nomad for those single days - i thought about doing that but never did!