documentclass{article}
% usepackage[notablist,nofiglist,nomarkers]{endfloat}  %% places all figures and tables at the end.

% includeonly{Doc1}

begin{document}

include{Doc1}

include{Doc2}

end{document}

As I show in Figure ref{fig1} in Doc2 ...

begin{figure}
centeringrule{1cm}{1cm}
caption{This is a figure}
label{fig1}
end{figure}