This inserts the number of page only for one author like "(Author1, 2003, p. 11)"
However, I would like something like "(Author1, 2003, p. 11, Author2, 2003, p. 22)"
The optional argument of the natbib citation commands --
You could achieve what you're looking for by issuing the command
Note that the output may be more easily parsable by your readers if you use a semicolon rather than a comma between the two