Ok... thought this is a simple scenario but it isn't. I spoke to RTI tech support and it gets complicated when you have K4's that need to communicate with the RP6. So luckily I have a couple CB8's included. So what I need to do is connect the "Loop Out" of the CB8 to the "Expansion" on the RP6. Now if I want RF control I will also need to connect the "Signal and Ground" as Drewski300 states above. Statement below from RTI Tech Support:

"Basically, a zigbee remote is not compatible with an RP6.

Therefore if you have 433 remotes, you can wire together the signal in to signal in and ground to ground between the processors.

If you have in-wall controllers, they need the CB4/CB8 output hard wired to the expansion port of the RP6 and IR control will flow over to the XP8. (NOTE: the xp8 expansion port is NOT tied to the phoenix connector like it is on the RP6). Or the output of the CB4/8 could be run to both expansion ports – one from the output, one from the loop out.

Two-way controllers RK3/K4 use Ethernet to control the xp8…so in theory they only need connection to the RP6 if needed."

I'm not using any zigbee just K4's for 2way and T2c's and T1b's.