I haven’t got to grips with a matte – can I (say) use it to block-off half the scene with a chroma colour which would be used for keying.
two of these (top and bottom) could then be used on two identical clips to move the clip apart and reveal a bottom layer holding a “second” clip. The motion would be customised.
Can a “blue screen” matte be sized and overlaid to effectivly create an alpha channel.
The only other way I see is creating two clips separately with a chroma fill on each and while they remain in their normal position they result in a seamless join (doubtful) until each move. The upshot then is the end clip is third generation, not to mention the storage they would need.
Slinks away to look at the help file…………… (Assuming it is understood)