Adding a second amp to factory amp

[InkedShanty]

I bought my 2005 M&T Vibe a couple months ago, and finally moved my subs and amp from my past car. I already replaced the HU with a JVC KD-S26, and wired everything else up like a normal install. Power wire through the firewall, RCA cables from the HU, etc. My Kenwood Excelon KAC-X521, doesn't seem to supply enough power to the subs- a pair of Dual 12" They are working, but not pushing it the way they were in my old car. All the bass is coming from the one 8" Monsoon.Is there some special wiring needed to add an aftermarket amp and still use the factory amp?