Do you want both subs to be used as LFE subs. If your receiver has only one LFE out then you would have to split between the 2 subs using a Y connector but this would reduce each by 6dB as a result so you would have to crank the volumes at the subs. You can also use the LFE out for one sub and then wire the second sub from the receiver to the sub using speaker cables and then from the sub to the front speakers using speaker cables. The same can be done with the surround speakers but this would in effect create a dedicated surround sub for the rear channels only.
Some subwoofers these days also have an LFE OUT which allows you to daisy chain subs together. If you haven't gotten one yet and would really like the benefit of two subs then that may be an option to look for.
Or, you could get passive subs (like an SVS CS series) and and amp like QSC or Samson or any amp that has a parallel input mode. This allows you to run the one cable from your receiver but send the signal out of both channels in the amp. Therefore you power both subs with one, untainted, LFE signal.
Thanks for another method. I was going to use the term daisy chain in my original question, but I didn't know if anybody would even know what that meant anymore.