Is it possible to have multiple images side by side?
No, the editor will only allow one image per row.
The philosophy behind MageMail is to focus on simplicity and encourage best practices for building a newsletter that will look great across a variety of devices - mobile, tablet, and desktop.
One of the reasons that single-row images tend to work well is that they resize down nicely for mobile devices. There are ways to have multiple images per row in a responsive layout and we may support that at some time in the future, but for the sake of simplicity, we aren't supporting that yet.
One workaround you could use in the time being would be to simply create a single image with both of those two images side by side, and then upload that as a single image.