If the subs are different models or sizes don't install anything.
use the best subwoofer you have, if you use both you can have cancellation with uneven woofers or damage them. use a large ported box and tune it close to the music you listen to.
use a minimum of 4 gauge copper or cca wire. anything smaller and you risk burning the wire, the brand doesn't matter. use 14 gauge or larger for speaker wire and setup your impedance of your subwoofers in the pattern that will give you more power.
see here.
http://www.the12volt.com/caraudio/boxcalcs.asp