I've added the event Sprite1 on collision with Sprite2 and the sub-event Sprite1 Y = Sprite2 Y.
The action is Destroy Sprite2.
Shouldn't walking with Sprite1 into Sprite2 on the same platform (same height) trigger the event?
Example:
http://dl.dropbox.com/u/35440583/Height_Test/Height_Test.capx