How useless is the passenger side mirror on the 69 911?
 

Seriously...I can't even see it when I'm sitting in the driver's seat. It's completely obscured by the A Pillar and the window frame. Bad design?

Any tricks, or should I replace it with another mirror?

that's because you used the oe holes in the door matching the driver side , right?
so yes it is now basically useless

for install examples go look at any 911 SC or carrera with power mirrors- you will see where they placed the passenger side mirror

There have been a few threads previously about the fact that the passenger mirror is in a different location - it is mounted more to the rear. You'll have to do a search to get all the correct dimensions, but here is one that came up:

http://forums.pelicanparts.com/showthread.php?s=&threadid=196453&highlight=durant +mirror+passenger

yes different location than OE pre drilled holes which were there for RHD, factory installed second mirror was farther back and the actual mirror was more convex than driver mirror (durant mirror). I believe there is an actual factory location for that mirror that you can find on the door, it is referred to as a divot or indentation under the door skin.