I recently started working on a DJ tool that can link 2 c64's together via serial cable. Here's a video of my progress through November:
As well as sync the running tracks can be manipulated by an fx chain with snapshots, if you've used VSTs like Stutter Edit you'll get the idea. When a track has been setup it can be saved with the config back to disk so you don't need to redo your work when playing a new set.
I did have previous demos using multiple SIDs to run tracks on the same machine, however from my research most people's dual SID setups don't have indivudal outputs. Instead if a second SID exists it can be used for echo effects or routing FX out of the chain early.
