I'm proficient in Qwerty, Dvorak, and RSTHD [1]. I also learned Colemak at one point and was proficient at it, but learning RSTHD replaced my Colemak muscle memory (which in itself is odd and interesting). Right now I use RSTHD when I have my ErgoDox, Dvorak on a standard keyboard, and Qwerty on mobile. Some additional things I've found:

* It's possible to be proficient in multiple layouts at a time as long as you use all of them on a regular basis.

* Furthermore, it's possible to separate your muscle memory for a layout from your muscle memory for the physical keyboard enough to be able to switch between keyboard layouts basically at will.

* Getting used to moving around punctuation is much harder than getting used to moving around normal letters for some reason.

* If you use different keyboard form factors (for me, ErgoDox vs. standard keyboard), your muscle memory for the layout is tied to your muscle memory of the physical keyboard itself. I can't type Qwerty on an ErgoDox but I can just fine on a standard keyboard.

[1] https://xsznix.wordpress.com/2016/05/16/introducing-the-rsth...