I've found it. I had to wade through the ATP Rulebook but it's something that has bothered me for a while so it didn't bother me. Here's the extract:
C. Doubles Main Draw - ATP and Challengers
Direct acceptances for ATP and Challenger Series events shall be in the following order:
1) Teams composed of players in either the INDESIT ATP Entry Ranking (Singles) or the ATP Doubles Entry Ranking. The combined positions of the two players (using best singles or doubles Entry Ranking) shall be added together and the total used to determine the Direct Acceptances. The most recent INDESIT ATP Entry Ranking (Singles) and/or ATP Doubles Entry Ranking List, including any Entry Protection, shall be used.
Ties shall be resolved as follows:
a) Team using two (2) singles Entry rankings.
b) Team using one (1) singles Entry ranking.
c) A team using an Entry Protection is positioned below the other team(s); if one player on each team is using an Entry Protection then the position of each remaining player determines which team is positioned higher.
d) For ties between teams with the same composition, priority will be given to the team with the strongest individual singles Entry Ranking, including Protected ranking, or for teams using two doubles Entry Ranking's, the fewest number of tournaments played and then the team with the highest number of points.
2) If the doubles draw cannot be filled by teams with at least one member who is listed in the ATP Singles or Doubles Entry Ranking,
a) ATP - the remaining places shall be filled with byes.
b) Challengers - the remaining places shall be filled, at the option of the Tournament, with unranked teams or with byes.
D. Seeds -ATP and Challengers
Seeded teams will be determined by using the combined doubles ranking of the two players (entry protection not included). Ties shall be broken as follows:
1) The team with the fewest events played is positioned higher.
2) The team with the highest number of points is positioned higher.
3) A coin flip or draw if a tie remains.
Qualifiers are teams who are included in the main draw as a result of their success in a qualifying competition. If the main draw is made prior to the completion of a qualifying competition, it shall include as many qualifying places as there are sections in the qualifying draw. Each of these qualifying places shall be positioned in the main draw in accordance with standard drawing procedures. Determination as to which qualifier goes into which qualifying place shall be by drawing after the qualifying competition ends.
Quite a shame that the rule actually seems to favor the singles guys over the specialists, which is a shame. But the seedings is doubles ranking, which is fair.
Found a couple of other fascinating thing out about the particular rules concerning qualifying. Oh, and did you know the top 10 aren't allowed to enter challenger events at all? And events are only allowed to give WC's to a certain number of players from 11-50?
Anyone else interested in this lengthy document you can find it here