You definitely should get an SPL meter if you don't have one already. You need that to set the gain on the back of each sub to match each other.
You might want to read this discussion level matching vs. gain matching
and then the Audioholics dual sub setup guide
if you haven't already. It is possible to manually generate the frequency response graphs mentioned in the article. Many people do it by hand with Radioshack SPL meters. Or you could buy a calibration mic and learn to use REW
. Then there is the XTZ room analyzer
if you want something automatic.
As to where to set the crossover on the AVS? No lower than the low frequency roll off of the speakers, but you might want to go higher depending on what sounds better to you. It is often recommended that you go no higher than 80hz since above their, the bass frequencies can become more localizable.
Now, I could be wrong, but if your subs are equidistant from the listening position, then I think they should probably be in phase with each other (verify that). So set the phase on each to zero. Then Audyssey will adjust the timing for the sub output to match it to the rest of the speakers.
Your SVS manual will tell you which RCA input to use on the subs for hooking them up to the LFE output on your receiver.
Finally, you are kind of in the wrong place to ask these questions. AVS has a dedicated subwoofer forum
where there are many people with experience with dual subwoofer configuration that can help you.