In the demo, the double jump is more demanding. In the final version, we've increased the time window during which you can perform it. It's not available for the entire time you're in the air, but it's definitely more forgiving and easier to use.
We don't like having a double jump available at any time while airborne; we think it feels a bit awkward, and too many modern neo-retro games use it that way nowadays. That's simply how we see it.