Dash + double jump will get you further rather than using one of them, and dash doesn't work vertically unless you are using jump - then your full speed will be horizontal speed multiplied by dash plus vertical jump speed (that will give maximum reach).
Also, in the second level the platform with the key is unreachable unless you use both jump and dash mechanics and your shadow mechanics. I know some players managed to find workaround :D by climbing outside of the level. I will change the position of the key in this level in the next update :D