How to enable the button only when all targets are dragged?

[Utaiweb_Wu]

Dear flash masters,

My question is how to go to go to previous/next frame after ‘monkey’ and ‘dog’ are both dragged out of the scrollpane.

The file is placed below:
https://www.sugarsync.com/pf/D9726155_61875891_24948

Thanks for any advise.