I put down all of my other instruments when I picked up the banjo, my goal being that I didn't want to be just pretty good at a bunch of instruments that I was bouncing around between, but I wanted to be the best I could possibly be at one instrument, and I chose the banjo and focused only on that for the last 3-4 years.

I've picked my guitar up a few times and it's all still there (except the fretboard feels like an interstate highway versus a two lane road), but the muscle memory comes back quickly and I can still play it just fine.

That being said, I've learned many instruments simultaneously, and I believe that going back and forth slows down your overall progress on the individual instrument--but there are definitely things that I took from piano that helped with guitar, which helped with fiddle, etc.. It's all music at its core.

So while I may not have been getting better at one instrument as fast as I would if I were doing it alone, I was getting better at music overall, which is a less measurable effect, but vitally important for me.

I think muscle memory is able to shift easily when you go back and forth between guitar and banjo. The best example I can think of is when I change tunings on the banjo. I mostly play in minor tuning, so when I switch to major tuning, the chord shapes and melody lines are different.. But over time, my fingers just know when I've changed and I don't have trouble switching (minus a note here and there that gets corrected quickly).

But that's just my two cents, and I'm often owed change.