The double jump is frustrating to play with as it only works at the top of your jump. You should be able to double jump at any time in the air... doesn't feel good at all.
Viewing post in Metal Miracle (Game Boy Color) comments
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.