Stephane,
The Under-Seat subs run in stereo, so using a single "sub" channel for them might not be optimal. You would also have to run them in series or parallel to have both on one channel of an amp, which would do interesting things to your impedance rating.
In case you aren't familiar with what it does:
http://www.hometoys.com/content.php?...2Fspeakers.htm
Using a 5-channel amp in our cars is problematic. You may want to consider the Helix 8 Channel or the Bimmertech version of it.