Are there other cities in the world like Boston where a large number of good postsecondary institutions are located?
So, "good postsecondary institutions" is a bit hard to define. University rankings are probably about the only thing less meaningful than impact factors, but they'll do to give us a basic idea here - do any cities have a large number of universities that are rated higher than the lowest one in your set?

The US News &amp; World Report has a handy list searchable by city. Annoyingly it doesn't quite work for the US (it defines city strictly) but it's a good approximation. Brandeis is rated #280 (making it fifth in the Boston area, if you include Northeastern).

On the US News &amp; World Report ranking, and plugging in a few likely candidates, there are


six London universities ranked higher than Brandeis.
five NYC universities ranked higher than Brandeis.
four Paris universities ranked higher than Brandeis.


So that's one higher, one equal, one close. In the specific case of London, Wikipedia has a handy table of universities by ranking. Depending on which metric you want to use, it has either five, six, or eleven (!) institutions ranked higher than Brandeis.

So with these admittedly flawed tools, it looks like London is likely to be an answer, but there are a couple more close contenders. There may be other concentrations in the US depending on how generously you define the city areas - I would guess the Bay Area around San Francisco is a likely candidate, for example.