https://github.com/diamondburned/slemmy does but it doesn't work with anything above 0.18 version at the moment. There is a list of different frontends listed in this blog post.
Lemmy
Everything about Lemmy; bugs, gripes, praises, and advocacy.
For discussion about the lemmy.ml instance, go to !meta@lemmy.ml.
Firefox containers? (that’s what I did to manage my alts on Reddit)
Not quite what I mean, sorry. Let me explain
All Lemmy instances are seperate websites. wefwef.app is a front-end for Lemmy where you can add multiple accounts to, but the UI is not good for desktop use at all, even though all features do work.
Sorry I didn't explain that very well. I don't think there is a desktop client that supports switching between multiple accounts, but as I say you can sort of achieve the same goal by using Firefox containers. Each container has separate cookies so if you have one container for each account you can just switch container to switch account.
I know that's not quite what you're asking for, but I'm not sure what you're asking for is possible.