From Nate Ewell, Director of Media Relations for the Washington Capitals.
Bring any used or unused D.C. United game ticket to the Verizon Center for the Capitals home game this Sunday at 5pm vs. the Ottawa Senators and you will be given a complimentary ticket to the game.
D.C. United ticket(s) will remain in your possession and must only be presented to receive a ticket to the Capitals game. D.C. United tickets will not be collected or scanned.
On Frozen Blog 
I’ve seen this somewhere else as well – where is this info coming from?
MCW
I wish I had saved my ticket from the United game earlier this year.
I know for a fact that there is some cross-promotion going on between the two teams this weekend. And it’s great to see this sort of thing happen between the Caps, Nationals, and United.
(I have no idea if Redskins or Wizards or other teams are involved in this sort of thing.)
MC, there’s now something about it on the frontpage of washingtoncaps.com