As far as I'm concerned there are a few options, most of which you mentioned. Which one you choose depends on how critical of sound quality you are and how much you are willing to work/spend.
1) Use the splitters and split the Subwoofer out RCA jacks to power the subs. IIRC your head unit only has 2V preamp outs (assuming that detail on Crutchfield is accurate) so this might not be the best option for SNR and sound quality but then again it might not be terrible either... this is definitely one of the easiest ways and cheapest wasy to go about this project.
2) Use the signal from the rear speakers. I know some companies make a device that lets you tap right into the rear speaker signal to power the subs. IIRC the device passes the rear speaker signal through a low pass filter and sends it to your amp. Not my favorite option as I prefer to use a dedicated preamp out for my subs... but I've known people who did this and it worked fine.
3) Use the rear speaker RCA outputs on the back of your deck, run those through a low pass filter, and to your amp.
4) Does either of your amps have a passthrough option? When designing my system (still haven't purchased all the parts yet... just designed it) I made sure that at least one of my amps had a preamp passthrough. This would allow you to run the SW-R and SW-L outputs from your head unit to one of the amps, then run 2nd amp off the preamp passthrough outputs on the first amp. It pretty much just passes the signal through so you can chain the amps.
2005 Windveil Blue GT (MT)
93H Bama tune, JLT Series 3 CAI, FRPP CMDPs, Pype bomb super system, 3.73 gears, Steeda shift bushings, Saleen short shift block
Steeda ultralite springs, Tokiko D-specs, J&M Extreme joint UCA/LCAs, PHB, C/C plates, bumpsteer kit, Steeda Q1S wheels, 2011 GT brakes, 275/40R18 Sumi HTR Ziii tires