Compare the cost of living between X and Y (cities)

That's a nasty error. Mountain View is stored internally as "Mountain View, California" and therefore the usual form of Mountain View does not match. Moreover, this error also shows that at least in this case partial matches are not recognized properly, as they should.

Needless to say, both issues are now on my list, and I'll fix them as soon as I can (I'm now on my "day job", expatistan.com is a side project).

In the meantime, as a workaround you can still use the site if you start typing 'Mountain View' and wait the two seconds that it takes the autocomplete to suggest the city. Then click on the suggestion and you should be set. Alternatively, you can just use this url the first time, and then change the second city on the boxes at the top right:

http://www.expatistan.com/cost-of-living/comparison/seattle/...