I'm 'fluent' in QWERTY and DVORAK. Learned QWERTY when I was 13 years old, learned DVORAK around college years. For DVORAK I used Stamina Typing Tutor - https://typingsoft.com/screenshots.htm It was frustrating at first, but within a few months I only made mistakes with very rarely accessed characters.

Since then I've been able to switch between the layouts swiftly. At work I use Dvorak on my external keyboard, but when I take the laptop with me I switch to QWERTY. The switch is a mental flip akin to talking in another language (I'm fluent in Russian and English).

I think the switch is very worth it. If I was re-doing it, I'd probably learn Colemak instead of Dvorak (apparently it's a slightly better choice for some reasons).

When I moved to France I would use the Canadian French keyboard layout which was QWERTY-based with subtle changes and a whole bunch of modifiers for different accents above/below standard Roman letters. I was surprised at how quickly I adapted, even to the character variations. It took only a week or so to become proficient. While dvorak et al are certainly more different, I think it isn’t unrealistic to master those layouts in a week or two. Muscle memory is a remarkable thing.

I used to be fluent in QWERTY, Dvorak, Colemak, and Carpalx (i.e. >60 WPM on any of the listed). To learn each I did the cold turkey method the author seems to have settled on as well. The first alternative (Dvorak) took about 2 weeks to get to the same proficiency but I was using a desktop computer for ~16 hours a day during this time period. I followed up with Colemak to see if it really was any better and then heard of Carpalx shortly after that. I ended up liking Carpalx the most though it comes at the cost of not being preinstalled. The Carpalx website has the X11 keyboard definitions, I had to make a custom Windows keyboard layout install package for my work desktop.

Nowadays I'm only fluent in QWERTY and Carpalx since I stopped using the other two once I made my decision but switching is no different for me than when it was 4.

Anyways to answer your question now that the background is out of the way: Switching for me seems to be more a function of how comfortable I am with the layout than anything else by far and I've become extremely comfortable with the layouts I use. I can switch between QWERTY and Carpalx instantly and I very often do e.g. when a game was coded based on the letter sent instead of the scancode or when I'm in an RDP session to a prod box.

The only time things can get jumbled for me is if I'm working in RDP with QWERTY on one monitor and another layout locally. Jumping between I often forget it's a different layout because I haven't consciously switched by triggering the input method toggle keyboard shortcut like I normally would. The solution is simple though: If I'm going to spend a lot of time hopping between the RDP session I'll just toggle to QWERTY globally until I'm done.

I'm a swiper on phones so it makes sense to leave it qwerty since that's what the swiping algorithms target.

My experience was quite different to that described in this article. Around 20 years ago I switched to Dvorak on my personal computer. It took me about two weeks to get used to it, and maybe four more to gather up to the speed I typed in QWERTY. Throughout this I still used QWERTY on other computers, like the family computer and computers at university.

I had no trouble switching between layouts, and could type 60wpm in each, in consecutive minutes. I tested this when I had my computer in the family room connected to the family computer to transfer files (parallel cable!). I had one keyboard on the desk and the other on my lap and had no difficulty with the different layouts while working on both computers simultaneously.

I never got much faster in Dvorak than QWERTY, but it was much better for my hands on the keyboards of the time.

I personally pretty much ditched QWERTY for now (I use it only on my phone).

But I think it's possible to maintain 2 layouts in your muscle memory