Another solution to consider (although it only makes sense in certain scenarios) is to use a differential. It naturally creates a 2:1 ratio, but if your mechanism doesn't require one then it's kinda bulky and silly to introduce one with a fixed input. But if you have one involved, just swapping around how you're using the inputs and housing can introduce a 2:1 ratio where you didn't have one before.
I did this today designing a mechanism for a clamshell excavator. Turning one crank extends both lines at the same rate, turning another crank keeps one line fixed while adjusting the second. But I wanted a block-and-tackle on the suspension line, meaning it would need to turn twice as fast. Just changing how I used the differential got me that ratio for free.
Here's an illustration of the basic concept - the differential housing will rotate at half the rate of the yellow shaft. Like I said, silly and bulky, but could be useful in the right situation.