The best way with an RP6 would be to use a CM232 and do RS232 control of both zones (volume needs to be IR). I just took a look at the back of our demo MX136, and it has an A and a B IR input. I believe A to be for Zone A, and B to be for Zone B (I don't use zone B on these to often). I'm using IR for vol up/down, when I move from the A jack to the B jack, I lose control of Zone A, so I assume that B is for Zone B; but remember what happens when we assume.

After verifying this (because we had only assumed), use port routing in the RP6 to send the commands to the proper input.

I have also assumed that the MX134 is pretty similar to the MX136 in connection and operation. This is a relatively safe assumption, as McIntosh doesn't change up their control schemes very often. Hope this helps a bit.