I will let other people flame me......
I hate multiple casts in a macro for people that want them both to go off.
Here is how it works.
You cast a spell. It takes time to cast it. The server doesn't finish the spell yet and you can try to send another spell just before the cast finishes and it should go off. But that depends on your latency and reaction time. If you use almost all instants, it doesn't help. if you cast a LOT of long spells, and have a long time from your computer to the server, it can happen more often. I just don't plan on it. I play 1 WOW on a computer and have a fast connection, so it doesn't help me at all, the second cast NEVER, not in 110 tries ever went off. I got bored and pissed and tried it out.

The reson your casts get ganked, is your second cast tries to over write them. Put a [nochanneling] on the second part I think. I just put things in one cast and move on. There are bigger things to worry about than if you can squeeze out 1 extra cast due to lag.