A video layer can be split into two new video
layers at the frame you specify.
Select a video layer in the Animation palette.
Move the current-time indicator to the time or frame
number where you want to split the video layer.
Click the Animation options icon and
choose Split Video Layer from the Animation palette menu.
The selected video layer is duplicated and appears
immediately above the original in the Animation palette. The original
layer is trimmed from the start to the current time, and the duplicate
layer is trimmed from the end to the current time.
Original layer before using the Split Video Layer command
Resulting two layers after using the Split Video Layer command