Decompose stereo signal
There a few commercial algorithms that do exactly that (Dolby Prologic, DTS Neo 6, Lexicon Logic 7, Bose Videostage, etc.). If your project has the funds, you can simply try to license one of those.

The inner workings of these algorithms are rather complicated and typically a mixture of time domain and frequency domain feature extraction, some steering logic, and some real time steering mechanics (time variant mixers, sub band filters, etc.)

Simple signals like your example are normally easy to tease apart, however almost all music is  partially correlated and overlapping both in time and frequency. For starters it's difficult to define what actually SHOULD be in the center and the Left and Right. The algorithm has to make a lot of "artistic" decision to make sure there are no steering artifacts and it meets the design intent over a wide variety of source material.