Crossfire works by alternating frames between the first card and then the 2nd card. Both cards use their own memory to render their frame. As a result, you effectively have only the amount of video ram as each individual card has.
What resolution do you play that flight simulator at? I find it hard to believe 1GB isn't enough. Of course I don't play flight simulators (or at least I haven't in nearly 10 years), so I might be out of the loop here.