Not one time have I written a macro like

/cast
/cast

as they were never supposed to work, if both are on the global cooldown. It is supposed to work if the first spell is not on the gcd.

Ya you can use isboxer to send multiple keystrokes (kinda "work around the rulz" so I dont use it, but legal from what I read).

Just have more steps .... gives you more practice pressing the same key so you can do it faster anyways. And you can trigger on the down and up cutting your presses in 1/2.