Here's a website put together by Josh Williams that allows students to analyze data from two different maps simultaneously. It's called geteach.com (and it's free!).
For example, let's say that you wanted to show students how different landforms/elevation effect population density. When you first open up the site, you'll see two maps, shown side by side.
To compare two different maps, click the "Select Map" button in the bottom right hand corner of each image and choose whatever type of map you wish to see. Down below, I've selected "Physical" and "Population Density" for students to examine.
There are many different types of maps available on the site. It's all quite cool!
There are many more "advanced" features available on the site for students/teachers to use, but I just figured I'd share some of the basics with you all.
Thanks, and enjoy!