Consider to switch to biblatex which features an advanced mechanism for name/name list disambiguation. In the following example, I tried to emulate the behaviour of the chicagoa style with respect to name list citations; however, additional formatting tweaks may be advisable for the bibliography.
\documentclass[a4paper,12pt]{article}
\usepackage[style=authoryear-comp,natbib=true,maxbibnames=99,maxcitenames=2,
uniquelist=false]{biblatex}
% Remove "In: " for articles (courtesy to Herbert)
\renewbibmacro{in:}{%
\ifentrytype{article}{%
}{%
\printtext{\bibstring{in}\intitlepunct}%
}%
}
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@article{author1,
author = {Author, F. and Author, S and Author, T},
journal = {Journal A},
title = {First Paper},
year = {2000}}
@article{author2,
author = {Author, F. and Author, S and Author, T and Author, F.},
journal = {Journal B},
title = {Second Paper},
year = {2000}}
\end{filecontents*}
\addbibresource{\jobname.bib}
\begin{document}
Some words \citep{author1, author2}.
\printbibliography
\end{document}
I'm not aware of a BibTeX style file for PNAS, but the Bibulous project does provide an easy way of customizing styles. For the style suggestions linked to by the OP, it took me only a few minutes to put together a complete style template to follow PNAS' requirements. Using the following main.bib database file
@ARTICLE{Neuhaus,
author = {Jean-Marc Neuhaus and Liliane Sitcher and Meins, Jr, Frederick and Thomas Boller},
year = {1991},
title = {A short C-terminal sequence is necessary and sufficient for the targeting of chitinases to the plant vacuole},
journal = {Proc Natl Acad Sci USA},
volume = {88},
number = {22},
pages = {10362-10366}
}
@INCOLLECTION{Hill,
author = {Adrian V. S. Hill},
year = {1991},
title = {HLA associations with malaria in Africa: some implications for MHC evolution},
booktitle = {Molecular Evolution of the Major Histocompatibility Complex},
editor = {Jan Klein and Dagmar Klein},
publisher = {Springer},
address = {Heidelberg},
pages = {403-420}
}
and the style template file main.bst (the lines below show the complete file)
TEMPLATES:
article = <au> (<year>) <title>. \textit{<journal>} <volume>(<number>): [<startpage>--<endpage>|<startpage>|<eid>|].[ <note>]
incollection = <au> (<year>) <title>. \textit{<booktitle>}[, vol.~<volume>, ][, <edition_ordinal>~ed.][, <null.if_singular(editorlist, edmsg1, edmsg2)>~<ed>][, <series>][, Chap.~<chapter>] (<publisher>, <address>)[, pp~<startpage>--<endpage>|p~<startpage>|<eid>|].[ <note>]
SPECIAL-TEMPLATES:
authorlist = <author.to_namelist()>
editorlist = <editor.to_namelist()>
authorname.n = [<authorlist.n.prefix> ]<authorlist.n.last>[ <authorlist.n.first.initial()>][<authorlist.n.middle.initial().compress()>][, <authorlist.n.suffix>]
au = <authorname.0>, ..., <authorname.9>
editorname.n = [<editorlist.n.prefix> ]<editorlist.n.last>[ <editorlist.n.first.initial()>][<editorlist.n.middle.initial().compress()>][, <editorlist.n.suffix>]
ed = <editorname.0>, ..., <editorname.9>
null = {}
OPTIONS:
edmsg1 = ed
edmsg2 = eds
compiling the main.tex file
\documentclass{article}
\usepackage[paper=letterpaper, text={6.5in,9in},centering]{geometry}
\makeatletter %
\renewcommand{\@biblabel}[1]{#1.}
\makeatother
\begin{document}
\nocite{Neuhaus,Hill}
\bibliography{temp}
\bibliographystyle{temp}
\end{document}
produces the following formatted result:
This provides templates for only journal articles and articles/chapters in books, but the PNAS website provides guidelines for only these two. Templates for other entry types are easily derived from the two shown here. (For example, a book entry type template can be defined by adding another line
book = <au> (<year>) <title>. ...
in the lines below TEMPLATE: in the style template file.)
Best Answer
There is no 'standard' way of indicating this information in a
.bibfile. Moreover, it probably would be difficult to include it in theauthorfield without breaking standard styles/sorting/... As such, if adding this information I would use a custom field (the BibTeX format allows for this as all non-recognised fields are ignored).