\documentclass{article}
\usepackage{xr}
\externaldocument{test2/input1}
\externaldocument[1-]{test2/input2}
\begin{document}

It would be great to be able to use omnicomplete in this file in order to
reference equations, e.g.~\ref{eq_1} and \ref{1-eq_2}.

\end{document}
