[Plugin: NextGEN Gallery] Displaying two images per page in imagebrowser

I would like to display the next image alongside the current image shown in the imagebrowser.

I think I need to create a new variable. This is the code that sets up the URL for the image for the current image:

$this->imageURL = get_option ('siteurl') . '/' . $this->path . '/' . $this->filename;

What should be changed to generate a URL for the next image coming up in the imagebrowser rotation?

Thank you.