Including two separate figures in latex as a subplot

Question

I have two separate figures in tex file: fig1.tex and fig2.tex for example

\begin{figure}[t]
\begin{tikzpicture}
.
.
.
\end{tikzpicture}
\caption{caption}
\label{fig1}
\end{figure}

I use \input{fig1.tex} and \input{fig2.tex} to include them in main.tex as two separate figures

How to include them in one figure as subplots?

Answer 1

The trick is to temporarily redefine figure to be subfigure.

\begin{filecontents}{fig1}
\begin{figure}[t]
\begin{tikzpicture}
\node{Hello world!};
\end{tikzpicture}
\caption{caption}
    \label{fig1}
\end{figure}
\end{filecontents}
%
\begin{filecontents}{fig2}
\begin{figure}[t]
\includegraphics{photo}
\caption{caption}
    \label{fig2}
\end{figure}
\end{filecontents}
%
\documentclass{article}
\usepackage{graphicx}
\usepackage{tikz}
\usepackage{subcaption}

\begin{document}
\begin{figure}[t]
\bgroup
  \renewenvironment{figure}[1][]{\subfigure{\dimexpr 0.5\textwidth-0.5\columnsep}}{\endsubfigure}
  \input{fig1}\hfill
  \input{fig2}
\egroup
\end{figure}


\end{document}

Answer 2

It is not entirely clear what you after. Probably something like this?

This can be simply to obtain by use of minipages, in which you insert for example tikzpicture by use of \input{<file name>} and some image on the same way (however, in such cases is simpler to insert it directly by using includegraphics). Of course, each minipage should contain \caption and \label too:

\begin{filecontents}{fig-a}
\begin{tikzpicture}
\node{Hello world!};
\end{tikzpicture}
\end{filecontents}

\begin{filecontents}{fig-b}
\includegraphics{photo}
\end{filecontents}


\documentclass{article}
\usepackage{graphicx}
\usepackage{tikz}

\begin{document}
\begin{figure}[ht]
    \begin{minipage}[b]{0.45\linewidth}
\input{fig-a}%\includegraphics[width=\linewidth]{example-image-duck}
\caption{caption}
    \label{fig-a}
    \end{minipage}\hfill\begin{minipage}[b]{0.45\linewidth}
                          \input{fig-b}
                          \caption{caption}
                          \label{fig-b}
                         \end{minipage}
\end{figure}
See figure \ref{fig-a} and \ref{fig-b} ...

\end{document}