Arduino - Multiple button event with lcd shield
Depends how the buttons are wired, is it in a matrix, or is each button individually readable? If it's individual, I don't see what the problem is, just read both buttons, wait until they are both pressed, and then wait two seconds making sure they don't get un-pressed during that time.

If they're wired in a matrix, it may not be able to read both of them at the same time.