If I were doing it, I’d start with a click track. That helps me immeasurably until I get enough pieces going to hear the feel.
As far as the dueling arpeggios, it could get a bit busy, but you don’t know until you try. Doubling parts can be nice. Try it out and see if you like it (it is your recording). If seems too busy, my thought would be to have something like the GTR doing full time arpeggios and the banjos sprinkling arpeggio highlights and phrases above it.
One last thought: Don’t be bashful about doing something like starting really stripped down and developing the instrument mix as the song progresses.
Looking forward to hearing it!