Have a variable in the combined table named City. Rows in the combined table that came from your New York table would have NY in that variable, and those that came from your San Francisco table would have SF in that variable. Once you've added that variable to each of your tables,join them together using City as your key variable.
Once you have that as a variable in your table you could use it as the grouping variable in calls togroupsummary if you want to compute, say, the total population for all the regions / boroughs / etc. in your table that are in NY.