My questions exactly.
The entire point of the BGS, according to what they say, is to provide a good experience to the user (with "big world" data) without it having to connect to multiple places, so no, I don't think it will be possible to connect to multiple BGSs.
In fact for most of the time the user won't even connect to the BGS, it will connect to the other things that read from the BGS, the feed providers and the labelers.