Separating two frequency component
You asked "how" without first asking "if", and, if so, under what circumstances.

If you add two completely unknown numbers A + B and get 100, can you un-mix the sum to the original two numbers A and B?

But if you know B = 7, you might have a solution.  

In the receiver case, if you know the 2 spectrums are disjoint (or other properties, such as redundancy in modulation/coding, etc.), judging might be possible.  Or the answer might be statistical.