Originally Posted by
airzim
It's not really practical. Even if you disembark in Term B (where LH is) you still have to clear C/I and then recheck security to go back into the terminal with the dreaded Term B TSA agents..
Or clear security at C and then take the bus to B. And when you're done with the LH lounge, walk/take train to whatever terminal the DCA flight operates from. Lots of people do it, so I wouldn't call it "not really practical".