Allow for playing multiple pieces at the same time

[deuill]

I've found that certain pieces combine very well, especially ones that provide soundscapes when played along ones that feature distinct notation (e.g. Eyes Closed + Transmission). Thus, it might be interesting to allow for playing multiple pieces at the same time, and perhaps evolve this at some point for synchronizing key/beat and so forth, while also extending to recording features.

[alexbainter]

I'm getting started on a big ol' redesign right now and I've been thinking about including this, so it's good to see more interest. Thanks!

[edelstone]

+1