Q:What if your target ceases to be a legal target between the two attacks produced by Twin Attack?
A: If, for some reason, the target ceases to be a legal target after the first attack, the second attack is lost, just as it would be if the target were defeated.
www.wizards.com/default.asp?x=starwars/a... - it's the very last question.
What if the target was never legal in the first place? If bombad gungan makes you target someone you normally couldn't target would you still get the twin attack on that target?