They aren't 100% segregated though, there was the officer exchange program in FS2.
Somewhere in the BP fiction it said that the exchange program all but stopped. Found it, it's in "The Rift". They obviously aren't excluding one another (ex: Vasudans in Sol, and I'm sure Vasudans travelling through Terran worlds), but when you evacuate Capella, there's a line about getting "every last Terran" out. So they either didn't care about Vasudans (not too probable at that point) or the system did not have a large enough quantity of Vasudans to mention.
I'd actually characterize their relationship much more like units of a federation that don't like eachother all that much. Being a Canadian the example that comes to mind for me is the East-West political divide/rivalry.
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That might be a more refined example, but I couldn't say about Canadian politics. We have enough fun here in the States.

So, what kind of system is the GTVA? Is it like the EU, where it's a mostly economic union (correct me if I'm wrong), or is it more like the two sides are states/provinces united under a certain code and follow the GTVA leadership? The FS2 mission 1 brief makes it sound more like the latter, but by the time the GTVA enters Sol in BP, it looks less so.