As others have said, it’s fine, as long as you’re not using it to ban evade or otherwise break instance or community rules.
Also, come to think of it, even if it were okay, would it even be possible to have to be logged into two separate accounts on the same instance at the same time with the same browser profile? :/
Yep. But you need to use an alternative front end or a 3rd party app for that. If you try alx.lemmy.blahaj.zone for example, that’s our Alexandrite front end, and you can have two or more accounts signed in with Alexandrite