I don't think you need more than one; I use TotalFinder right now and love it but I haven't ever really looked at PathFinder. I guess if you can afford it and there are certain features that each have that you need; if not then there's no reason.
I have been using totalfinder as well, but I am finding it becoming a little annoying in terms of usability .. I really like that you can have "bookmarks" in pathfinder ... I don't know