Natbib: Multiple citations with page numbers in one bracket

It can be automatized, but the basic is like this:

\begin{filecontents*}{\jobname.bib}
@article{adams03,
 author={A. Adams and F. Fournier},
 title={?},
 journal={?},
 year=2003,
}
\@article{collier09,
 author={C. Collier and S. Someone and T. Tomeone and V. Vomeone},
 title={?},
 journal={?},
 year=2009,
}
\end{filecontents*}
\documentclass{article}

\usepackage[american]{babel}
\usepackage[utf8]{inputenc}
\usepackage{natbib}

\begin{document}

\citep{adams03,collier09}

\citep[p.~3]{adams03}

(\citeauthor{adams03}, \citeyear{adams03}, p.~3; \citeauthor{collier09}, \citeyear{collier09}, p.~5).


\bibliography{\jobname}
\bibliographystyle{apalike}

\end{document}

A better version, with a syntax slightly different from \citep, but easier to manage for multiple citations:

\begin{filecontents*}{\jobname.bib}
@article{adams03,
 author={A. Adams and F. Fournier},
 title={?},
 journal={?},
 year=2003,
}
\@article{collier09,
 author={C. Collier and S. Someone and T. Tomeone and V. Vomeone},
 title={?},
 journal={?},
 year=2009,
}
\end{filecontents*}
\documentclass{article}

\usepackage[american]{babel}
\usepackage[utf8]{inputenc}
\usepackage{natbib}

\usepackage{xparse}
\ExplSyntaxOn

\makeatletter
\NewDocumentCommand{\multicitep}{m}
 {
  \NAT@open
  \mjb_multicitep:n { #1 }
  \NAT@close
 }
\makeatother

\seq_new:N \l_mjb_multicite_in_seq
\seq_new:N \l_mjb_multicite_out_seq
\seq_new:N \l_mjb_cite_seq

\cs_new_protected:Npn \mjb_multicitep:n #1
 {
  \seq_set_split:Nnn \l_mjb_multicite_in_seq { ; } { #1 }
  \seq_clear:N \l_mjb_multicite_out_seq
  \seq_map_inline:Nn \l_mjb_multicite_in_seq
   {
    \mjb_cite_process:n { ##1 }
   }
  \seq_use:Nn \l_mjb_multicite_out_seq { ;~ }
 }

\cs_new_protected:Npn \mjb_cite_process:n #1
 {
  \seq_set_split:Nnn \l_mjb_cite_seq { , } { #1 }
  \int_compare:nTF { \seq_count:N \l_mjb_cite_seq == 1 }
   {
    \seq_put_right:Nn \l_mjb_multicite_out_seq
     { \citeauthor{#1},~\citeyear{#1} }
   }
   {
    \seq_put_right:Nx \l_mjb_multicite_out_seq
     {
      \exp_not:N \citeauthor{\seq_item:Nn \l_mjb_cite_seq { 1 }},~
      \exp_not:N \citeyear{\seq_item:Nn \l_mjb_cite_seq { 1 }},~
      \seq_item:Nn \l_mjb_cite_seq { 2 }
     }
   }
 }
\ExplSyntaxOff

\begin{document}

\citep{adams03,collier09}

\citep[p.~3]{adams03}

\multicitep{adams03, p.~3; collier09, p.~5}.

\multicitep{adams03, p.~3; collier09, p.~5}.

\multicitep{adams03; collier09, p.~5}.

\multicitep{adams03; collier09, p.~5}.


\bibliography{\jobname}
\bibliographystyle{apalike}

\end{document}

Different keys are separated by a semicolon, a postnote is separated from the key by a comma (if a comma is in the postnote, place it between braces). You can actually use it for a single citation, like

\multicitep{adams03, p.~3}

Also consider the \citealp syntax described at http://merkel.texture.rocks/Latex/natbib.php

You still need to bring your own parentheses and semicolons, but the syntax is slightly more compact than above (modified code below):

\begin{filecontents*}{\jobname.bib}
@article{adams03,
 author={A. Adams and F. Fournier},
 title={?},
 journal={?},
 year=2003,
}
\@article{collier09,
 author={C. Collier and S. Someone and T. Tomeone and V. Vomeone},
 title={?},
 journal={?},
 year=2009,
}
\end{filecontents*}
\documentclass{article}

\usepackage{natbib}
\begin{document}

\citep{adams03,collier09}

\citep[p.~3]{adams03}

(\citeauthor{adams03}, \citeyear{adams03}, p.~3; \citeauthor{collier09}, \citeyear{collier09}, p.~5).

(\citealp[p.~3]{adams03}; \citealp[p.~5]{collier09}).

\bibliography{\jobname}
\bibliographystyle{apalike}

\end{document}

Here is an alternative solution:

\citetext{\citealp[p.~1]{ref1}; \citealp[p.~2]{ref2}}

gives

(ref1, p. 1; ref2, p. 2)

In your case:

\citetext{\citealp[p.~3]{adams03}; \citealp[p.~5]{collier09}}

gives what you want.