Satisfying this (silly) sorting scheme requires using some advanced features. You should use a custom field to sort by and use a source map to populate the field with a construction functionally determined by the number of authors. What we are doing here is effectively calculating the number of ands in the author name list which, due to the BibTeX name format, is a count of the names. So, a concatenation of the ands is a string which can be used to sort the entries. Then, a custom sorting scheme sorts on this field before alphabetical name sort:
\documentclass{article}
\usepackage{csquotes}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@article{AuthorA2010,
title={Titlea},
author={Author, Alan},
journal={Journala},
volume={10},
pages={1--3},
year={2010},
publisher={Publishera}
}
@article{AuthorAC2005,
title={Titleac},
author={Author, Alan and Cuthor, Carla},
journal={Journalac},
volume={10},
pages={1--3},
year={2005},
publisher={Publisherac}
}
@article{AuthorAD1990,
title={Titlead},
author={Author, Alan and Duthor, David},
journal={Journalad},
volume={10},
pages={1--3},
year={1990},
publisher={Publisherad}
}
@article{AuthorAD2000,
title={Titlead two},
author={Author, Alan and Duthor, David},
journal={Journalad two},
volume={10},
pages={1--3},
year={2000},
publisher={Publisherad two}
}
@article{AuthorABD2000,
title={Titleabd},
author={Author, Alan and Bill Buthor and Cuthor, Carla},
journal={Journalabd},
volume={10},
pages={1--3},
year={2000},
publisher={Publisherabd}
}
\end{filecontents}
\usepackage[style=authoryear,firstinits=true, dashed=false,sorting=custsort]{biblatex}
\addbibresource{\jobname.bib}
\DeclareSortingScheme{custsort}{
\sort{
\field{presort}
}
\sort[final]{
\field{sortkey}
}
\sort{
\field{usera}
}
\sort{
\field{sortname}
\field{author}
\field{editor}
\field{translator}
\field{sorttitle}
\field{title}
}
\sort{
\field{sortyear}
\field{year}
}
}
\DeclareSourcemap{
\maps[datatype=bibtex]{
\map{
\step[fieldsource=author]
\step[fieldset=usera, origfieldval]
\step[fieldsource=usera, match=\regexp{.+?\sand\s}, replace=\regexp{and}]
\step[fieldsource=usera, match=\regexp{((?:and)*).+}, replace=\regexp{$1}]
}
}
}
\begin{document}
\nocite{*}
\printbibliography
\end{document}