Nick Do you mean an effect, or a transition? A transition would be doable I think. Though, there are a few issues to deal with (whether the left or right channel is the one that the keying would be applied to).
samiam i guess it would be an effect that works like a transition, or whatever way would work best for your to have a moving shape cut out with a green background and then mix that over the top of the other deck so its super clean.
