2 mic recievers from the same company, shouldn't their carrier frequency be the same?.
Each set would probably have a mic that is 100MHZ and a mic that is 105MHZ, or else it would be impossible to use them at the same time.
Each reciever recieves signals from 2 mics so 2 recievers will be for 4. Each one is connected to a line on a mixer so the output from the 4 mics is mixed and sent to the speakers but the problem again is that both don't work together : one works and the other doesn't. if you switch of the working one, the other works and the previous doesn't work again if you switch it on later. Hope my question is clearer now? and are your answers to them the same or do you see something else?
Do two mics come with each receiver that you have, like this:
Set 1. Receiver, mic, mic
Set 2. Receiver, mic, mic
If they are the same exact sets, then this will happen:
Set 1. Mic1-100MHZ, Mic2-105MHZ
Set 2. Mic1-100MHZ, Mic2-105MHZ
As you can see, if mic2 from each set is turned on, you'll have trouble because both of them are on the same carrier frequency. The same will happen with mic1.