I was able to find some clarification on the rules in qualifying, though not specifically for this event. The key part that helped me track this information down was the "Advance-47/On-Site-54". Based on the information from this document(page 93-94), there are qualifying places designated for those who have signed up for the alternate list for a given tournament. Additional places are made available when other qualifying teams withdraw prior to the start of the qualifying rounds. There's a lot of solid information about qualifying in section 7 of this document.
B. Main Draw Doubles
1) ATP Tour Tournaments
a) Advance Entry. 12 Noon,
Eastern Time, USA, fourteen (14) days prior to the Monday of the
tournament week (or the first tournament week in the case of
tournaments scheduled for more than one (1) week). A team on the
alternate list for the doubles main draw may withdraw at any time
prior to such time 94 VII. THE COMPETITION that they are moved into
the main draw as a direct acceptance because of withdrawals of other
b) Withdrawal Deadline. The deadline for a team to withdraw
from the advanced entry list closes at 12 Noon, Eastern Time, USA, on
the Friday prior to the tournament week.
c) On-site Entry. The period
for online on-site entries opens at 12:01 am, Eastern Time, USA on the
Friday prior to the tournament week and closes at 5:00 pm Eastern
Time, USA on the Friday prior to the tournament week. The online
sign-in is blind and no preliminary lists will be produced. Players
entering multiple events in the same week must do so with the same
partner and shall designate tournaments by priority. The number of
places available through the on-site entry is specified in the
Composition of Draws (7.08.B) plus any spots made available through
withdrawals occurring after 12 Noon Eastern Time, USA, on the Friday
before the event week.
d) Limit to On-site Entry. A team is allowed
entry through the on-site entry method a maximum of two (2) times per
year in events where they would have been accepted, using their
doubles ranking, on the original list in that event through the
Advance Entry system, if they had entered.
Another point worth considering -- I found this quote from a New York Times story on his return to play. Probably helps to call the owner directly to secure your spot in the qualifiers.
“They called us and said Andy really wanted to come back to D.C., and wanted to play doubles with his brother,” said Mark Ein, the new owner of the tournament.
Further, the Citi Open website clarifies the numbers of each type of qualifiers.
Main Draw Doubles Direct Acceptances: 10
Teams Wild Cards: 2
Teams On-site Entry: 3
Teams Qualifying: 1