Two keys on one press is ok, if they're modifier keys.
It's also ok, if its two spells on one key, but with a press per activation, such as a cast sequence.

It's not ok to do 2+ actions on one press, with delays.
I could create (via my G15) a macro which will: Immolate, Agony, Corruption, Drain Life... with a 2 second delay between each spell, activated on a single keypress.

Basically, if you could accomplish your macro within Warcraft, using their macro system its ok.
If you could not do your macro within warcraft, but can via G15, Keyclone, IS, HKN etc, I'd refrain from doing so.
But its your account, make your own choice.




*Edit*
I've read in your (Zanthor's) IS thread a bit.

I can create a key within warcraft which is my assist key.
This is not a spell or ability, so does not trigger a GCD.

If I were to really quickly press this key and then a DPS key, that's possible within the warcraft client.

You can press one key on your client and have it send the assist press followed by the DPS press.
That's a gray area, since you're only pressing once, but are receiving the benefit of two clicks.
I'm not singling IS out here, Keyclone does the equivalent with the Hotstrings for the FTL system too.

Still, from the games perspective, they are receiving an instruction to use the assist method of choice.
Followed by the DPS spell of choice.

I personally think this is ok from Blizzards perspective.
Only one game spell/ability which impacts the game environment is pressed on a single keystroke.
It makes no difference to Blizzard if you were to have four assist keys, and spam them sequentially and then cast one spell.
It does not enable bot actions, and you're at the keyboard for the whole time you're playing.