Onkyo TX-SR875 on board AMP + Pre out together?

I use the Onkyo to drive my 5.1 HT speakers but have a need for the front signal of the Pre to go into a separate amp to drive another pair of speakers, all playing the same source at any given time. Can I do that or do I have to choose between the onboard amp or pre out?
I think the Onkyo sends both a pre-out and amp-out simultaneously, so you shoudl be able to do that.
The difficulty will be getting the phasing right on your 4 "front" speakers.