A Well-Known Principle

9 5
A K J 2
A K J 8 3
A 5
Q 4 7 3 2
10 8 6 Q 5 4
Q 10 7 5 4 2
Q J 9 8 6 2 10 7 4
A K J 10 8 6
9 7 3
9 6 Contract: 6 spades & NT
K 3 Opening lead: Q of clubs

This hand should be made in both spades and no trump. No, not because one can drop the doubleton queen of spades. I would have taken that finesse the same as most declarers. No, it should be made for another reason, one that is fairly well-known among experienced players, to wit: You have a finessing position in two suits. You can make if a finesse works and you're down if it doesn't. What to do? If they're both on or both off, it doesn't matter what you do. What if one works and the other doesn't? Is there a magic formula? Not exactly, but you can increase your chances, you can indeed oftentimes resolve the problem to your satisfaction by picking the suit you'd like to finesse in, then first go for the drop in the other. And if it's going to work, it doesn't matter which one you pick first.
Hence: you wanna finesse in diamonds? Cash the top hearts and see if you drop the queen, and not dropping it, now take the diamond hook. You can see what would happen. Oh, you wanna finesse in hearts? Then let's go for the drop in diamonds, and bingo. The contract is ours.
Further, in a spade contract, you not only might go for the drop in diamonds, which we can see would work, but you might even ruff out an off-sides queen. I note that one declarer who went down in NT took the opening club lead in dummy, then the nine of spades was pushed through to the queen, took the next club lead, and having an entry back to the hand could go to his red cards where he now cashed the ace of each! It's not that that in itself harmed him, but if he would have just stuck with one suit and then come back to run spades and finesse in the other, he would have had it. But instead of cashing the king of diamonds before finessing in hearts, he came back finessed in hearts and was down one.
Another declarer just wasn't watching. Playing from the other side of the table in spades (here the no trump (North) hand played it in spades on a transfer, while the spade hand played it in NT as above), this declarer had the diamond suit handed to him -- and didn't notice. Opening lead was a low diamond, the queen drawing the king. Finesse into the queen of spades, 10 of diamonds drawing the king. At that point he held J 8 3 and a defender held 7 5 in that suit. Even if he wasn't good at watching the spots, if he'd just noticed that the jack of diamonds was high, he would have had his contract. But declarer insisted on ruffing out his diamonds, including a ruff of the 8 with the 7 played under it and fourth hand already showing out! And if that wasn't enough, he still could have made the hand if he'd cashed his jack of diamonds instead of finessing in hearts! He then would have had 5 spade winners, 3 diamonds, two clubs and two hearts.
Declarers who don't pay any more attention to their winners than that have to learn to watch the fall of the cards along with, or maybe prior to, all the hoary bridge maxims.