Will, Kirk told me exactly the same thing - driver's side only, mirror gets mounted on door; both side mirrors, mirrors get mounted on fenders. I wanted mirrors on both sides, so they were mounted on the fenders.
What I found is that these mirrors, with the standard flat glass, are small and designed for being used close to the driver (on the driver's door). If you mount one way out on the driver's fender, it's too far away for you to see a wide enough angle of view through it. What I ended up doing is ordering the convex glass that Marty mentions from Sierra Madre for the driver's side. Besides being of a higher quality finish, the wider angle of view lets it function much better as a useful mirror. With yours mounted on the door, you may not need to do that.
The one on the passenger side is a joke. Yes, I can see it even with the side curtains in, but it's so far away, it's like looking through a telescope (this is why the passenger mirror on normal cars is always a convex mirror). I put one of those cheapie NAPA wide-angle mirrors on it which turns it into a fisheye reflector. It's still almost worthless, but at least lets me see cars that are very close and in the blind spot.
If your mirror is already mounted on the door, I would leave it there and drive it that way. You can always switch in the wide-angle convex glass later. And if you drive top down most of the time like I do, you may not need a mirror on the passenger side.
You can always add one later, choose the glass you want, and place it exactly where it works for you.