How about posting the code for the receiver.
Also post a copy of the output, and explain how it is not what you expected.
My gut feel on this is that the data from the two transmitters stands to get mixed up, unless measures are taken to prevent it. The best approach might be to change to API mode. It can probably be made to work in transparent mode, but some scheme to prevent the two transmitters from transmitting simultaneously will be needed. Such an implementation stands to be relatively clunky as compared to API mode.