When setting up wireless microphone systems each microphone that will be in use will need a dedicated receiver (ie: one microphone to one receiver). Both the microphone and the receiver must be on the same frequency in order to interact with each other. It is not possible for two microphones to be sending a signal to a single receiver at the same time.
If the two microphones were set two different frequencies they cannot both be picked up by the same receiver as a receiver can only be tuned to one frequency at a time hence the receiver will only pick up signals that are on the frequency that it is tuned to.
If two microphones are used on the same frequency at the same time then they will interfere with each other because the receiver cannot distinguish between different signals and cannot separate them out into separate signals. In a situation like this neither signal will come through correctly. A good way to visualise this is to think of the microphone transmitters as two different radio stations and your car radio as a receiver. You can only listen to one radio station at a time because the radio can only be tuned into one station at a time. You can also have only one station broadcasting on each frequency or neither gets through clearly. If you've ever travelled and had two radio stations overlap on the same frequency with neither coming in clearly, you've heard an example of this.
NOTE: When a product appears to have once receiver receiving multiple microphone (like the Sennheiser EM 2050) it is actually a situation where the product contains two receivers built side by side into a single housing.