The warcraft keyboard API allows for key press and key release events to be registered. A few addons currently take advantage of this (Macaroon, Snowfall). You could have your castsequence sent on either press or release, with your IWT sent on the other.

For those who don't want to do an action on press and another on release, you can have your software alternate between IWT or the castsequence key, both on release (which is the warcraft standard), and then spam your key a little faster then normal.

Warcraft uses the GCD to limit what can happen simultaneously on a single input. All of the FTL assist systems (can set up FTL with almost all software options) do an assist and a spell/ability on a single push, and no one has been suspended or banned for this.