If a pair of floats are to be forced to form a 2-page spread (in a book, or whatever), the first must lie on the left side of the spread, on an even-numbered page. The dpfloat package provides for this: the construction to use is:
\begin{figure}[p]
\begin{leftfullpage}
<left side figure>
\end{leftfullpage}
\end{figure}
\begin{figure}[p]
\begin{fullpage}
<right side figure>
\end{fullpage}
\end{figure}
The construction has no effect unless the standard class option
twoside is in effect.
Full documentation of the package (such as it is) is to be found in README.dpfloat