@Vegas If you don't mind explaining a bit, perhaps with a few pics, how you got your A600 and C64 running together with MIDI and MSSIAH, I'd be eager to learn.
On a side note, my C64 did not sell ironically, not even a bid and there were 11 watchers. I guess that is a sign that I should keep it.

As for the Second SID upgrade, yes it's totally worth it, you get 8 Channels instead of 4!
Having Dual POKEY on the Atari 8-Bit ain't a bad idea either, now all we need is something similar to MSSIAH for it, and we'll be set! The POKEY is a highly under rated audio chip and they are cheap!