There are two letters in each square! I'd like to thank OneLook for its extensive help in this endeavor. I couldn't have done this without it. (Inspired by a lot of other puzzles that've been going around lately. You should call yourselves "Mario," 'cause ya just been 1-upped)
Thanks for playing! Make sure to check out my other puzzles :)
Nice work! If you want a shortcut, check out Qxw - free software to make crosswords, in which you can specify the number of letters for each cell. It has many other handy features!