OK, let me make sure I have this straight...

-you have (2) 4-Ohm Infinity Perfect 12.1s wired in parallel to produce a 2-Ohm load
-you have an amp which will put 390W RMS into 2-Ohms
-a capacitor of some sort (you didn’t specify)
-This is in a VW Golf but might/will be moved to either a RAV4 or CRV in the not-too-distant future

OK, just so you know where I’m coming from, let me tell you what I have:

-(1) 4-Ohm dual voice coil Infinity Perfect 12.1 with the VCs wired in parallel to produce a 2-Ohm load
-an amp which puts 400W RMS into 2-Ohms (supposedly peaks @ 600W)
-no cap
-this is in the trunk of a Mazda MX-6 (not a huge car, but a bit bigger than your golf.)
-this is in a sealed enclosure that is just a bit smaller than infinity's specs. (What kind of enclosure are yours in?)

Let me say this puts out plenty of bass for my tastes. The perfects are power hungry subs and if you’re using a sealed enclosure they will want even more power (and will be able to handle it) ported enclosures are more efficient power-wise but the sub is more likely to be damaged by to-high power levels.

RAV4's and CRV's aren’t that much bigger than your golf (at least not so much that I think it will make a huge difference)

really, it sounds like you might be happy with just one of those subs and more power going to it. If you could find an amp that would put ~400W into 4-Ohms you would probably be surprised at what that one sub could do. I have infinity kappas up front with 100W running to each one and I have to keep the sub turned down just a bit to keep the bass from drowning them out. I have the gains set so that there is no clipping up to 0 db. With the windows up I rarely turn the volume past -15 dbs. You can’t listen to it at full volume with the windows up.

My point basically is, unless you’re trying to put cracks in the side walk, 2 of those w/ 1000Ws going to them is overkill. That said, right now you are defiantly not getting out of them what you could. If you are dead set on keeping both of them, then ~800Ws is what I would put to them. It will be loud, but you could set the gains low, which is always better than setting them high.

Also, setting the gains properly is very important to getting the most out of your system. I highly recommend this site for instructions on how to set them properly:

http://totoro.efiregate.net/HTMLPages/Tuning/GainSetting/GainSetting.htm

Also, I keep mine crossed over @ 85 Hz so power isn’t wasted trying to produce the higher frequencies, set yours depending on how low your fronts comfortably play, but I personally don’t think these subs sound very good above 100 Hz. I have the sub playing below 85 and the fronts playing above and everything blends nicely.

As always YMMV but I hope this helps and good luck!