I have a job with a RP1, T2C, and now a U1. The U1 is slightly out of range.

Anyone know of a way to extend the range.

I would add a RM433 antenna in a remote location if I could, but my understanding is that you cant add them to the RP1.

I have a cat5 and RG6 I could use simehow, that run from where the RP1 is, to where the U1 will be used.

Any ideas, or sucess stories?

Can't really add to the range. What you can do is move the RP-1 closer to the out of range remote. You can drive a much longer cat-5 cable to the connecting block. (1000 ft) I think.

I thought I read somewhere that you could use 2 antenna.??

There was a thread about using a cable splitter...

I have used an RG6 with bnc ends to remotely mount the antenna to a more middle location?

Although using RG-6 might extend the range of your RP1, I think that the use of RG58 would be more appropriate. I haven't found the specs but for my experience, I'm 99% positive that the antenna impedance for the RP1 is 50 Ohms. Even when the use of RG6 might increase your range the impedance mismatch between the antenna and the cable and then between the cable and the base would cause a signal loss compare to the same setup with a 50 Ohms cable.

I've had success with extension of a Pronto's RF range by making a passive extender. You put a pickup antenna on one end of a piece of cable and a transmit antenna on the other end. The antennas are the same.

Cut about a 10" piece of RG6. Strip back and expose about 7" of bare center conductor. Put an F connector on the other end. This is a quarter wave stupid inefficient antenna that will help you a lot.
Now make another one.

Put one of these on each end of that spare RG6, with the one nearest to the RP-1 really near to its antenna -- maybe a foot or less from it. The other end is to be in the area where the remote will be used.

When a Pronto would not go the whole sixty feet of a spec house in Malibu, I did this with the remote antenna behind drywall near a plasma. The remote then had about eighty feet of coverage.

In another location I plugged the pickup antenna into a 10 dB amp for even better range.