In short, no, not currently.
There have been plans since before Photosynth was released in August 2008 to link existing synths together which neighbour each other or overlap each other, but this is one of the more difficult problems that the Photosynth team has to face. This is still different to what you are describing, but could achieve many of the same goals.
People have been asking to be able to contribute their photos to existing synths for just as long as well as people who are actually on the Photosynth team. =) Unfortunately it is simply a question of resources and priorities. The Photosynth team is not very big and they don't really make Microsoft much money so it's not like they can ask for many more people to build all of these great ideas. It takes time to build each part and test it and make sure that each piece is solid before they release it.
The geo-alignment system that was released last fall is a big step forward towards linked synths.