I think that it is widely believed that it is impossible to add two seaters and I think this is little more than a rumour or a mistranslation. ED has now said that there will be one flyable added after 1.1(Ka-50) and that is it.

Reasons they were reluctant to add two seaters could very well be because they would only really work online and they were unsure of the games online potential. With two seaters they have to make two 3D cockpits for a single aircraft and usually more complex avionics as well.

It has been demonstrated however that flyable two seaters are very possible in Lock On there is a mod available at http://www.lomacfiles.com that allows two players to fly an Su-27UB trainer with a LAN connection.