What lar3y said is a good idea, but we can extend it a bit.
If after clicking on a format to display all the books with that format, you CTRL-click (hold the CTRL key down while clicking) on another format, all books for both formats will be displayed. In the search bar will be something like:
Code:
formats:"=EPUB" or formats:"=MOBI"
The above show all books with either EPUB or MOBI formats.
If you remove the "or" from that line and press enter, this will change to doing an "and" between the two search conditions. So, it will only show books with both formats.
You can type the search in directly if you want and you know what the search terms are.