Daniel is correct. The X-10 IR command set is not housecode specific. The housecode is set by the IR543 when it receives an X-10 IR signal from any transmitter, including a Pronto.
The RF command set it different though. Both the transmitter (i.e. remote) and the receiver are house code specific. You could plug-in 2 RF transceivers in the same room or different rooms, set them to different housecodes, and use two different RF remotes set to different housecodes. This would allow control of two houscodes in the same room, but using different remotes.
As Daniel mentioned, there are better ways to do this using a home automation controller. A home automation controller monitors the AC power line for X-10 signals. You can program them to "listen" for a specific sequence of X-10 codes and then have them react by turning on lights on any housecode. For example, you might create a button in a Pronto panel with X-10 IR codes 1,2,3 (not followed by on/off/dim/bright). Then set up a macro in the home automation controller to trigger a response when it "hears" 1,2,3 on the power line. Using this type of set up does not restrict the devices to 16. In fact, you can have as many macros as your home automation controller will handle. I use a Pronto and one IR543 to control my HVAC system, over 50 lights and lamps, appliances, irrigation, to querry a weather report, listen to voice mail, and more. This is accomplished with a Stargate home automation controller. In addition to the Pronto, these functions can also be accessed by wireless RF remotes, wall mounted LCD control panels, any using any telephone in the home.
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