It would be really handy if you could have two instances of renoise open and be able to simply copy/paste pattern sequences, instruments and settings between the two, it would streamline many processes
Don’t think it would work between two instances of Renoise without using the universal clipboard anyway, as each of them is running seperate. A neater and probably easier way to do it would be to have the option to be able to open more than one song at a time in the same instance of Renoise and tab between them.
Suggested several times. Has my vote!
I think a simple clipboard would do, no need to complicate things. A tab interface would involve to much work for the devs i suppose.
It would be a lot of work to give Renoise the capability of opening and playing multiple rns files simultaneously, and switching between then for editing. The tabs themselves would probably be the easiest part 
A universal clipboard would indeed be handy
Yearh. See Copy Buffer and Copying And Pasting Between Two Running Renoises for earlier discussion. I’m for the universal clipboard suggestion too.