Huge fan of switching instruments. Even when I think I’m doing fine on one instrument and I don’t need to do this, I sometimes begrudgingly have to pick another instrument because it’s the only one I have available. And many times, when I do that, I learn something. And I’m not talking about fancy advanced things, I’m talking about basic stuff like where do I place my hand and how do I make a certain motion work with smoothness.
In general, I find myself switching between 22.5" scale, 24" scale, and 25.5" scale guitars, two different size acoustic bodies, strat bodies and the Les Paul carved body, multiple bridge types — with and without tailpieces — and mandolin, F-style, A-style, and classical bowl back. It’s quite a variety. Honestly, I’d prefer to play one instrument all the time and be done with it, but I’m almost certain I’m better off with all the moving around I have to do.
It’s very hard to predict when you will or won’t learn something by moving around, so it’s difficult to say how you should decide when to do this. I don’t know what the formula should be. I can say for sure that locking yourself into one instrument, one form, one motion, etc. especially if you’re in the learning stages, and especially if you’re not hitting it out of the park already, is probably a negative. It’s an extension of the old approach of hammering away at the same exercises and patterns day after day, very slowly, with no variation. It just leaves no room for natural variety that all of us had when we were kids and trying everything and learning everything.
Edit: To be clear, I’m not saying you must change instruments all the time. I know that’s not always practical. I just mean that if you’re trying to acquire a new motor skill, there should probably be some source of variation, even if it’s little things like changing up your form, musical material, pick type, sitting vs standing, etc. Lots of little things you can do to help yourself feel what is common, and thus functional, about all these ways of playing.
And here’s some more mandolin!