Personally, I think the simplest solution is to restrict Prime Shot to rangers that take the archery path. A genuine TWF ranger doesn't really need Prime Shot, so I've got no problems preventing them from taking it. I do wonder if it's a significant enough ability to make TWF look less attractive, though. My next thought is maybe to increase the Prime Shot bonus to +2 when attacking your Quarry (+1 in other circumstances, although, if the closest target isn't already your quarry, the ranger would normally designate the target as such before taking the shot). That, IMO, would make the archery path a no-brainer for a character that was serious about being an archer. It may be a little too powerful that way, though.