How do I choose a double pedal?

You can adjust each double kick pedal to the height you want. They can hit at the same height or different heights. Generally, the beater will have a soft side and a harder side. You can set them so they're both soft, one hard and one soft, or both hard.

Are double bass pedals worth it?

So double bass pedals are crucial to your drumming for sure! But they can also suck up a lot of time when it comes to tweaking them so that you're able to play faster. If you're into that, that's fine. But be aware that it will cost you not only time; more features to tweak make for a higher price as well.