What DSP method are consumer products like the game Rocksmith and guitar polyphonic tuners likely to use for multiple f0 detection?
Check out chapter 1.3 of this IRCAM paper on multi-F0 estimation. It discusses the difficulties in extracting multiple F0s from a recording, including the handling of overlapping partials, transients, and reverberation, as well as the modeling of domain-specific sources with varied spectral properties.