Graphics, Figures & Tables ⇒ Images labels over two .tex files in Overleaf

[acnash]

Hi all,
I understand this has been covered several times, however, my efforts have failed spectacularly, and I would like some fresh opinions based on what I want to achieve rather than how I've gone about trying to achieve it.

I have a LaTeX project in Overleaf which contains two separate complete LaTeX document i.e., they have their own \documentclass, \begin{document} etc. They are called, main.tex and SI.tex and they can be found in the same file hierarchy.

I am simply trying to link a call to a label of an image in the SI.tex with the corresponding text in the main.tex. Using Overleaf I don't believe I have access/visibility to the .aux file; I understand that the xr packages reads data from that particular file.

SI.tex
-----
.....
\begin{figure}[h!]
\centering
\includegraphics[scale=.7]{QTAIM/2D_QTAIM_plots.eps}
\caption{Quantum Theory of Atoms in Molecules.......}
\label{fig:QTAIMSI}
\end{figure}
......


main.tex
-----
"..... Figure \ref{fig:QTAIMSI} ...."
......

Many thanks for your help
Anthony