to Richard Skinner --
your problem is the classic one that installers have learned to avoid by avoiding using two units of the same brand in the same room.

But there is hope! Xantech has an IR router that can be made to select which way to send different IR signals. You use this and put IR emitters over the receiver IR windows, then cover the emitters with black plastic so the IR will only go into the receiver you wnat it to go into.
As you might guess from this site, the thing to control it would be a Pronto or such. Here's how it works -- Let's say you make the main receiver the, well, main receiver. Then, every time you want to send a command to the second receiver, you send a macro --
a.Switch the Xantech to the second receiver
b.send the command you want to send to the second receiver
c.Switch the Xantech back to the first receiver.

This arrangement assumes that you use the controls of the main receiver more often. If not, then each command becomes
a.Switch the Xantech to the receiver you want to control
b.send the command to that receiver.

Any remote that can learn enough macros to do this will work transparently. If you had a remote that could just learn a couple of commands and were willing to cope with YOU always remembering how to do it, then you could teach the two Xantech switch commands to buttons on that remote.

Good luck!

Ernie