Depending on how you wire the subs, you will get different impedances (resistance). The lower the impedance, the more you get from your setup, but too low and your amp will blow :D
Find out what impedance your amp can handle.
Connect your subs to be as close to the lowest impedance as possible (without blowing up your amp).
If you wire the subs in series, i.e. amp - sub1 - sub2 - back to amp, then the impedance just adds up, so two four ohm speakers would make 8 ohms.
If you wire them in parallel, i.e. amp - sub1 + sub2 - back to amp, then the impedance is equal to the sum of the inverse of the speakers impedances. Rt = 1/R1 + 1/R2
This means you get a lower impedance. As it happens, if the impedences of the two speakers are the same you can just divide the impedance of one by the number of speakers. e.g. Two 4 ohm speakers in parallel would have an impedance of 2 ohms.
You can combine these to make different impedances. e.g. if you had 3 subs, you could wire two in parallel and then the third in series with those two. In this case, Rt = (1/R1 + 1/R2) +R3
Hard to explain without pictures. Take a look at this:
http://physics.bu.edu/py106/notes/Circuits.html
