As is standard, Enchanter. It's reason for the somewhat hillarious, out of the ordinary concept of a full group using 5 Summoners and an Enchanter. Or 3 Summoners and 3 Enchanters depending on how Behemoth works.
Personally, I like the idea of 5 Gnome Summoners and 1 Human Enchanter. If anything, for the spectacle of it all.
Aside for that, if only a duo, aside for Enchanter, any Healer would do if below the level to have Undine out, add to that, we don't know how good her Healing is. Further down the rung, I would say any Tank, but notably, Paladin if you decide to use a pet other than Titan and want to go more DPS focus.
Bard may be good if PRotF's Summoner and Bard are similar to EQ Mage and Bard. In EQ, the Mage's "weakness" (relative term) was be the lack of CC/camp splitting/pulling and that was the Bard's niche (especially if they get a slow and snare).
Another bonus in a duo like this is that gear would not be too competitive.
I can see a lot of potentinal with many of the classes in this game working well with the summoner. It all depends on the situtation. Like I can see a pushback ability and summoner putting up a wall to lock out mobs, if it is possible. Its going to be about what kind of creative play that will be allowed and what you come up with.