Ok...lets say that I have a Magic Drain and a Waboku face down on my side...my opponent activates Xing Zhen Hu on those two cards...Can I chain Waboku to his Xing Zhen Hu?

If yes, then what happens? Is his Xing Zhen Hu negated since it has to target 2 cards? or does his Xing Zhen Hu still work on my Magic Drain?

Oh yea...also if I have another face down s/t card...can my opponent switch to that one when Waboku is activated?

and if so, can I chain it...saying it's a Threatening Roar?

Yes you can chain Waboku........ Xing Zhen Hu will NOT be negated it will just try to resolve as much as possible........ in this case it will lock down Magic Drain

No he can NOT, Xing's effect targets so he has too choose the 2 cards first.......

As long as you didn't JUST set the cards, then Xing-Zhen-Hu will attempt to target your 2 f/d cards. Then you are given a chance to chain to its activation and able to chain Waboku and only Magic Drain will be locked down.

If you have another f/d card on the field, your opponent CANNOT "change his mind" when he sees you chained one of the one's he ALREADY targeted and select another card. Once the target has been selected, it cannot be "taken back."