Expansion content cost for multiple platform users.

So i went out and bought two copies of the ps3 version and two copies of the pc version so my wife and I would be able to switch platforms at will and still play our main toons. My question is about expansions. when an expansion comes will I have to do this all over again? (buy 4 copies) It would seem like I would not since what you are really paying for is access to new areas, new classes, and other additions. If I buy the expansion on only one platform and gain access to these things, then will I be able to log into the same account on the other platform and still have that access?

I know we have a ling bit of time before an expansion, but trying to decide whether or not to buy both copies for my son or just get him the ps3 version. Entertainment dollars only go so far...

I'm trying to remember how it worked with FFXI...but I "think" you had to buy both the Xbox 360 and PC copy to play on both. I might be wrong about this, as I didn't play very much on my 360, but I remember buying the all-in-one version for the 360 when I decided to play it on there at all. I know for sure you can't just buy the PC version and expect to play on PS3, but I think that's a given.