I recommend Liftoff! When you click on the three dots then you have the option to open the post on any instance you added.

I’ve got this in the feature list for Lemmynade—you can tap and hold your profile picture in the bottom-right for quick account switching at any time. This is awesome for times when you write a comment and then suddenly realize you want to post it under a different account (happens to me all the time!)

Sadly, the alpha version hasn’t been released quite yet so you’ll have to wait. You can follow !lemmynade@lemm.ee to know when it drops

I’m sure others developers will be implementing this down the road, we’re all working hard to get some good options out there for everyone!

Voyager sort of works for that. If you click into a post, switch accounts, and click on posts, then the post you were last looking at on the other account will still be open. However, if you misclick and don't tap on posts right away (e.g. tap inbox instead), then tapping the posts button will show the subscriptions sidebar instead of the post.