I am not sure what you are intending it for, but I have a set up sort of like this for my ret pallies and enh shammy for maelstrom/Art of War procs.

On my main cast bar key 2 is bound to button 2. Button 2 is my main dps click sequence for each toon. I found that if I added in a macro for /cast exorcism /stopcasting, I could cast exorcism whenever it was up but it would stall my click sequence if it came up without being off cooldown. Similar issue with the shammy except I used lightning bolt instead which did not have a cooldown, thus solving the problem.

For the Pallies I bound the exorcism macro to button 3 and key 3. In Isboxer I just added another step to my ftldps keys. Now when I spam key 2, on downpress, it sends key 2 to WoW but on up press it sends key 3. If exorcism is on cooldown, it does not stall the sequence, if it is off cooldown, it will fire 50% of the time.

I am not sure if you could achieve the same effect by bindind 2 key actions to the one step as I assume this would send the keypresses simultaneous. I am not sure if WoW would give some sort of precedence to one over the other or just not accept the keypress at all however.

Hope this is what you are after.

Cheers