I think your problem is that the receiver is receiving the signal from its own transmitter. This is the problem with trying to implement full duplex in any medium.
One thought, it might be possible to use a different modulation frequency for each path. Using a TSOP chip get one running at 30KHz and the other at 40KHz, that might give you enough separation.
However, it is much easier to implement half duplex, you still get two way communications, you just have to implement a protocol.