Hi guys
You can absolutely test more than 2 whole emails, but you need to be using the Email Program type.
Set up your whole email test, and select your first two emails (you need to click "Add" to add them into the test). Once you have two set up, Marketo will give you the option to add a third, and a fourth, etc.
Be careful that you are sending to a large enough sample size to warrant multivariate testing, and be VERY careful to only change a single aspect of your email (for instance, placement of call to action on the page). Too many variables does as others have noted make it very difficult to ascertain what was actually the successful factor in the test.
Have a look at this article for more info:
https://community.marketo.com/MarketoArticle?id=kA050000000LC9XCAW