If I understand this you have two emitters sending IR light to the same device receiver. Each emitter is connected to an independent source. The only time emitter #2 (AMX) is when emitter #1 (Xantech) is disconnected so it can not possibly emit IR light.

Try holding the #2 emitter inside your fist so no IR light can impinge on the receiver from that emitter. If the other emitter #1 now works then you know it is an IR light interference problem.

A xantech non amplified connecting block IIRC is passive. So it can not generate an IR signal only a source device connected to it can. An amplified Xantech connecting block can and in my experience often does generate IR noise.