I think no matter who you go into an alliance with you do need to get some rules sorted out first.
Firstly, what are going to be the rules for alliance chat? Usually full members have read and talk access, but I have seen instances where 2 guilds with different outlooks on what was approriate had difficulties when it came to spamming /as with drivel. So sort out what alliance chat is going to be used for (just information regarding rvr, just sorting out general PvE/PvP groups, or a social channel as well).
Secondly you then need to sort out what happens if people DO abuse /as. Usually they just get the right to talk in it taken away:-) but thats something you need to sort out first. You don;t want people complaining about each other in alliance chat, so get rules regarding problems with other guilds members sorted as well. I know this all sounds very serious, but it will happen, so it is best to get an agreed way of dealing with it.
You can;t really get the background of all their members (I am looking at YOU Beo :-)) as long as you both know what the alliance is for and what you want to see happen in the common chat channels, you have a good start! I think it is important that the two gms see eye to eye on the above points, and have a good working relationship.
Once you have an alliance up and running, others will see it and then it can start to grow. Having a strong idea from the outset of the type of alliance (pure rvr only, social, PvE etc) will make it easier.
Just my two pence, I used to go to alliance meetings in lieu of my old DAoC guild, so kinda know how these things worked back then, but ofc, things may be different now.(nah its the same hehe)
Edit: Also it is worth sorting out who decides stuff or if it is purely a democatic alliance, eg what if one of the guilds fancies another guild joining.....do all guilds get to vote, what if only one says no? Its really worth thinking about stuff like that.