If you are doing 5.1, then the surrounds should be to the sides of the listenning position......not 15' back(if that is what you meant).
Have a look at this:
http://dolbylabs.com/consumer/home_e...oomlayout.html
If using two subs is the way you want to go, then two of the same subs would be easier to intergrate than two different subs. Placement is dependent on room, acoustics, and what you are trying to achieve(ie, flat response vs. higher output).