How to get double column floating figure in IEEE rticles template from code chunk?

Question

I am using the "IEEE transactions" template in the rticles package for R Markdown. I want to have a 2-image figure that spans the entire page width. However, there seems to be no way of doing this from an R code chunk. Various errors occur, for instance when I try the "industry standard" (AFAIK) way of doing this for R Markdown:

{r TestGraphic, echo=F, out.width='49%', fig.show='hold'}
knitr::include_graphics(c("Test1.jpg","Test2.jpg"))

Inserting the above chunk into the standard IEEE rticles skeleton with no additional modifications, I get the following error:

! Undefined control sequence.
l.524 \includegraphics
                      [width=0.49\linewidth]{Test1} 

The resulting LaTeX code that is produced looks like this:

\includegraphics[width=0.49\linewidth]{Test1}
\includegraphics[width=0.49\linewidth]{Test2}

Which looks nothing like how the template describes multi-column figures:

\begin{figure*}[!t]
\centering
\subfloat[Case I]{\includegraphics[width=2.5in]{box}%
\label{fig_first_case}}
\hfil
\subfloat[Case II]{\includegraphics[width=2.5in]{box}%
\label{fig_second_case}}
\caption{Simulation results for the network.}
\label{fig_sim}
\end{figure*}

So the questions:

1. How do I get an R chunk to output the proper LaTex to give a "double column floating figure using two subfigures" as described in the template?
2. Is there any way to do this from a code chunk, or do I have to hard-code the LaTeX if I want to be able to resize images? 2.a. If it's the latter, what if I want to have a single figure with multiple subfigures that include both images and R-generated plots?