This is a hotly debated subject since subsonic sound is not directional. Having left and right subs based on the mono sub pre-out on most receivers will give a dual mono type-thing, and will result in hard-to-manage cancellations. If you can add a sub to each speaker-level wire and use the crossover in the sub to filter the bass, you may get a pleasing effect.
Most dual subs I read about here are right next to each other in a corner so that they boost each other with minimum interference.
If your subs are powered then perhaps they have speaker-level inputs/outputs with an adjustable crossover? If so, you can just run your Front Left ouput from your processor into one and Front Right into another (then wire from that sub to its respective main speaker) [this is the same thing Colin posted earlier]. This would give you stereo subs, but not necessarily the LFE channel of 5.1 material.
Some processors actually have multiple sub outputs--mine for example can handle at least four (maybe more; I forget) (Theta Casablanca II).
While sub output is generally considered to be non-directional, given the crossover points often associated with them you may find that they are in fact producing sound at frequencies that is in fact directional (above roughly 60 Hz). Some people I've spoken with have indicated that, in there systems, stereo subs (though they often use THREE--one for LFE, two for Left & Right) do give a somewhat different effect (better in their estimation) than running the same group of subs in mono. I don't have experience with it myself, though.