What you are asking is akin to saying after I mix my old sugar with a new bag a sugar how do I get them separated again? You can't. If you are satisfied with splitting the mono audio into two conductors then you can use a splitter but the right/Left channels will still be identical(ie. not technically right and left channel).
As Nicks Hinted at, you are not ging to get stereo sound out of a mono receiver.
This is because it only has mono sound demodulator/decoder tuner and other circuitry.
Your only choice is as Nicks said is to split the mono audio signal and feed it to both channels of the amp. Its just twin mono, NOT true stereo