Hi WESLEEPTHEYLIVE (woah!)
I don't know about the Kenwood, but the XAV-7W unit has 2 Audio/Video inputs so you can use one where you can connect the Front OR Rear RCA audio outputs from the Kenwood head unit, so you can listen to everything from both head units, using the XAV-7W as the master. You'll need a short stereo RCA cable to do this.
Of course you'll be playing back CDs in the Kenwood, but for the Radio stations, you'll need to decide which unit you'll use to tune them since your car only has one antenna lead. I would recommend using the Sony unit for AM/FM since that will allow you to see a little more of the station's info in the 7" monitor, which othewise would show you only AUX 1 IN or something like it if you use the Kenwood piece.
Think about mounting the Sony unit on the top spot if you don't want to have to close its monitor everytime you want to load/eject a CD in the Kenwood.
Actually, I understand you may need a kit or adapter to be able to mount 2 DIN-size head units, so do the research before you start tearing down the dash...
Your speakers would definitely will need to go connected to the Sony unit as long as you choose this option of the XAV-7W as the master.
Connectors wise, you can use butt connectors one size bigger so you can fit the two yelow, 2 red and 2 black cables from both head units to the same color cables on the harness adapter side. This may be a rude solution for some, so there's always the solder/heat shrink option, but I don't know if you're into that...
With this scheme, you won't need to connect anymore cables in the kenwood harness since the Sony will take care of that.
Don't expect the Metra harness to have an equivalent to the green wire in your XAV-7W; you'll have to run that directly to your ground wire in the hand brake base.
Last but no least, I'll recommend you paying an installer to do the job since this is no cheap equipment you got, and the installation can be easy but since you said you don't know about car audio, why risk it?.
Hope this helps...
Later,
FEDE.
