Here's a couple problems with that... 1) That Quart is probably just getting by, all by itself, on 4 ga wiring. The kicker is fused for 80 amps, and the Quart is fused for 140. You are running the quart at 1 ohm, so your are using it to its full potential and almost maxing out the capability of your power wire with just that amp. Adding that kicker amp to the mix will not be good in the long run. You can get by for now with a distro block on the end of the four gauge and eight gauge going to the kicker/four gauge going to the quart, but it would be best to either run 0 gauge if you want to run both off the same wire.
2) You said that the quart amp was grounded the same way. If it is, I don't even know how it's survived this long. Is the terminal for that sitting on top of the seat belt holder, like the other ground?