I have flown the UA / LH connection a few times and as soon as I have my reservation ticketed through UA, I use the UA provided LH confirmation code to log onto LH website and choose my seats.

Airlines do not usually have access to other airlines seat maps in my experience. Even when I used to book through UA.com flights that were operated by US Airways (pre split from *A), I would have to choose my seats through the UA Airways website with the US Airways conf code UA provided me in my itinerary.