It can be done. I can't speak for every install, but what I did today was hook up one antenna to the RG6 cable jack in one room. That line went back to the pannel. Then I tied that line to the RG6 that goes to where the RP1 is in the theater. I used a splitter as a combiner combining that line from the remote antenna, with the antenna for the local RP1, and hooled theoutput to the Rp1. Worked like a champ.

I've been wondering if that would work. I'm going to have to try that out sometime. I wonder what the limit would be for the number of antennas you could use.

Why not just use the RP-6 and a few RM433's??

I just did a simple RP1 install and the guy added a couple of outdoor speakers to his system and didn't get the best response from the remote out on the back porch. The RP1 was sufficient as far as IR control of his components while in the house, but it would have been nice to be able to extend coverage to the back porch. Looks as if maybe I could have.

The job was already contracted.

Why couldn't you add A RM433 to an RP1.

From the RM433 manual:
"Output signal is compatible with industry standard infrared repeating systems. This allows multiple IR and/or RF recievers to be wired together into an RTI control system."

I cant remember what connection the rubber whip antenna is. BNC? If so, did you just use a BNC to F adapter to connect it to the combiner?

Yep. Antenna BNC > F adapter > F Gender Changer > splitter