merge the guilds, or get every one to leave one guild and join the other, you would have to meet up on a planet both sides get to like Tatooine or Nar Shadda, and then have recruiters on both side. A Guild to Guild chat will NEVER happen, the demand is too low and the programming of linking two member only groups without them merging is too great.
Did you read what he posted? They have a Republic and an Imperial guild, both guilds have the same members. You cannot have both factions in one guild in this game.
And to the OP. I fully support this. We are in the same situation. And ever since we created our Imp side guild, we wished for such a feature. I don't know why they restrict PRIVATE chat channels that way.
We are a small guild, playing together for years, but most of us are altoholics who like to play both sides.