Unfortunately, you won't always have the X in your rack. The most common two-letter words are TO, IN, and OF.
It can score 26 or 28 points on triple square, so you might call it the "half X."
They are both good words to use to get duplicated tiles out of your rack to maintain better rack balance.
Two letter words can also turn a four-point word of all one-point letters into a 20-point play or more when you can hook three or four tiles at once. Even if you have the misfortune of having a fistful of consonants that include the Q, the Z, or both, you can still play these tiles by making either QI or ZA. There are no other two-letter words with Q or Z unless you rely on CSW, which includes words that are not in the OPSD6, such as ZE and ZO. Even with the extended word list of the CSW, there are no more 2 letter words with Q, so QI is it!
If you have two Vs in your rack, you're still stuck because there are no two letter words with V accepted in Scrabble. If you have all vowels, however, you can make AA, AI, OE, and if you count Y, AY. All of these can get excess vowels out of your rack and let you restore some rack balance. With the tactical use of triple and double squares, you could even score about 15 points, which is much better than swapping tiles for zero. Using your knowledge of 2 letter words with Q could even yield 66 points if you're extremely lucky!
In fact, your skill with 2 letter words is more important than having a large vocabulary. Many esoteric words require scarce letters, often multiples of such. You won't be able to make BACCHIC if any of the three Cs are played already unless you have at least one blank tile.
Only one of the 2 letter words uses no vowels: SH. If you use CWS as well as the OSPD6, then you can make words like CH, KY, NY, ST, and FY. That leaves 121 other words that use a vowel, and we all know that vowel tiles are the most common, with nine As, 12 Es, nine Is, eight Os, and four Us.
Speaking of Us, knowing how to get rid of them is another great skill to have in these word games. When it comes to two letter words with Q, you have QI, but you also have QAT, QOPH, QINTAR, QINDAR, QAID, and 10 others to use if you get stuck with a Q and have already ditched your U or Us. Just remember, however, that the Official Tournament and Club Word List, or OTCWL, is no longer applicable for games like Scrabble and Words With Friends. It was superseded two years ago by the NASPA Word List.
Knowing that there are no two-letter words with V is a good first step. So is knowing 2 letter words with Q boil down simply to QI. If you want to be a great word game player, know and memorize the list of two-letter words. They can be the difference between a win and a loss, particularly if your opponent doesn't know that there are no 2 letter words with V. If your opponent plays such a word, you can challenge and either resurrect a losing game or put a game where you are slightly ahead away with a well-placed bingo or single letter X.
As stated, TO, OF, and IN are all equally common, followed closely by IT, AT, and AN.