This code should insert another crawler onto your page;
The DD script supports multiple instances - the critical point being that a unique id must be used for each script-call, so Ive put those in and highlighted them in red. See how the same id is used once in the div that holds your train images, and then again in the script-call below.
<div class="marquee" id="mycrawler" style="" ><img src="images/shastadaylight.gif" alt="" /> <img src="images/sanfranciscochief.gif" alt="" />
inc: 5, //speed - pixel increment for each iteration of this marquee's movement
mouse: 'cursor driven', //mouseover behavior ('pause' 'cursor driven' or false)
Ive used the same code that you've used except that Ive inserted a unique id (the one currently on your page is called "mycrawler2") and I've also added the direction parameter (blue) so your 2nd train track will run the other way.
That's it for the implementation. You should just be able to paste that code into your web page markup and see a second train on the page once you refresh the browser.
Next comes positioning, so this is another time when you can use the style="" attribute inside of the div that holds your train images. I've put that into the opening div tag for you (green) so its ready to fill with your positioning CSS.