I am from a maths/physics background currently doing a PhD in fluid mechanics within the engineering department. I am not enjoying it as much as I'd liked and have raised this with my supervisor (only just, I am not sure what the outcome will be yet), and having done a little research into possible connected directions I can take it, I feel as though one of them would involve co-supervising with someone in the mathematic department, and one, if possible, by someone who is at a different university- but was at my university for around ten years, leaving around five years ago, and seems to be the only person who has worked in this area (it seems pretty niche worldwide relative to other areas) with connections to my university. Is the latter heard of for co-supervision or is it a very long-shot? many thanks in advance
It is not unheard of, certainly, though isn't especially frequent. But whether it can happen or not is up to your advisor and the university, as well as the other person, of course.
But you need to work out details in advance to make it successful. If it is typical for advisors to be co-authors on the resulting work (some fields) then you need to come to some agreement about that. Some compensation for the other professor might be available or not. It might even be needed.
But also be prepared for the distant professor to not do very much.