In TogetherJS both people are basically browsing like normal, but it communicates what's going on between the people (so for example you can see the other person's mouse), and synchronizes select things like forms. With the model of Surfly and Browser Mirror, one browser is the source, and the other is viewing what that other browser does. Everything "happens" on the source browser. This is in a session like this both people are logged in as the same person and seeing exactly the same thing, while with TogetherJS each person is logged in as themselves.
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It's more similar to an experiment that preceded TogetherJS, Browser Mirror: https://github.com/mozilla/browsermirror
In TogetherJS both people are basically browsing like normal, but it communicates what's going on between the people (so for example you can see the other person's mouse), and synchronizes select things like forms. With the model of Surfly and Browser Mirror, one browser is the source, and the other is viewing what that other browser does. Everything "happens" on the source browser. This is in a session like this both people are logged in as the same person and seeing exactly the same thing, while with TogetherJS each person is logged in as themselves.