Without in any way disagreeing with the answer that has been given and accepted, it seems to me that this is possible (though clunkily), if one assumes that all that one is really trying to do is, as the question asks, to allow \autocite to act as \footfullcite in relation to primary sources, and \parencite in relation to secondary ones. In other words, he wasn't really asking for two completely different citation styles, but simply for the use of variants (\footfullcite vs \parencite) perfectly proper to an author-year system which would be selected automatically by \autocite.
One might think that would be easy. But the difficulty is this. When a citation command declared with \DeclareCiteCommand starts up, it executes some precode. But that can't be used to decide whether to put a citation in a footnote or parentheses because at that point one can't "see" the data, which only becomes available as the citation's loopcode is executed, by which time it's too late! The solution to this I came up with involves defining a new command, not using the (proper) \DeclareCiteCommand method, which first runs a sort of "dummy" citation which just examines the citation data, and then pumps its arguments back into either \parencite or \footfullcite (for that purpose, since I'm terrible with optional arguments, I used xparse).
There are several bad things about this, as it stands. First, it's evading the proper interface for declaring cite commands, which is a problem for autocite. Secondly, it's not clever about punctuation or spacing: \footcite needs to close up space before the citation and move punctuation after it so it comes before. \parencite really needs to respect space before it, and to move at least some punctuation that is before it so it comes afterwards. Since the revised version doesn't move punctuation, the user still sometimes needs to know whether it's going to be primary or secondary data that gets cited, to get the positioning exactly right. At which point one asks: why not just do the whole thing explicitly? Lastly, there's a real problem knowing how to handle a single citation that contains both primary and secondary literature. I suspect the best solution would be to put it in a footnote, but with a full citation only for primary literature and a short citation for secondary literature. That could, actually, be done by redefining \footcite, but for present purposes I haven't bothered.
I daresay my approach is a poor one, and if I understood how \DeclareCiteCommand worked internally it might be possible to find a way to allow the precode part to peek at least at the first citation and take decisions accordingly. If that could be done then one wouldn't need to bypass the proper method for declaring citation commands, though some of the other problems would still be pretty difficult, I think.
\documentclass{article}
\begin{filecontents}{\jobname.bib}
@book{primary,
author = {Ancient, Albert},
title = {A Primary Source},
date = {1556},
keywords = {primary},
publisher = {Vetus Libris Emptor},
location = {Leiden},
}
@book{secondary,
author = {Modern, Mark},
title = {A Secondary Work},
date = {2012},
keywords= {secondary},
publisher = {Recentissima Societas},
location = {New York},
}
\end{filecontents}
\usepackage[backend=biber, style=authoryear]{biblatex}
\usepackage{xparse}
\makeatletter
\newtoggle{filtered@primary}\togglefalse{filtered@primary}
\DeclareDocumentCommand \filterandcite { o o m }{%
\filteredcite{#3}%
\IfNoValueTF{#2}
{\IfNoValueTF{#1}
{\iftoggle{filtered@primary}
{\unspace\footfullcite{#3}}
{\space\parencite{#3}}}
{\iftoggle{filtered@primary}
{\footfullcite[#1]{#3}}
{\space\parencite[#1]{#3}}}}
{\iftoggle{filtered@primary}
{\footfullcite[#1][#2]{#3}}
{\space\parencite[#1][#2]{#3}}}}
\DeclareCiteCommand{filteredcite}
{\unspace}
{\ifkeyword{primary}
{\global\toggletrue{filtered@primary}}
{\global\togglefalse{filtered@primary}}}
{}
\makeatother
\renewcommand*\autocite\filterandcite
\addbibresource{\jobname.bib}
\begin{document}
It is possible to have a system which will use keywords to decide
whether to put a source in a footnote wil a full citation, if it is
primary,\autocite{primary} or in the text with a label if it is
secondary\autocite{secondary}.
However: it's not a very robust system, because (a) it doesn't move
punctuation intelligently---so you still need to keep track in your
own mind of what is in the citation.\autocite{secondary} (A more
intelligent system would move that full stop past the citation.) And,
because it is redefining \verb|\autocite| outside \texttt{Biblatex}'s
dedicated mechanisms, it doesn't play well with \verb|\autocites|: as
can be seen here \autocites[10]{primary}[11]{secondary}. Finally---and
unavoidably---it's liable to `do the wrong thing' if an automatic
citation contains both primary and secondary
sources\autocite{primary,secondary} (though it's not clear exactly
what would be the `right thing' in these circumstances).
\printbibliography[title={Primary Sources}, keyword=primary]
\printbibliography[title={Secondary Sources}, keyword=secondary]
\end{document}