Several ways to do this, depending on how you need to composite the shot. If you need both masks at the same time, you can use Shape Layers. Instead of a mask draw and animate a Shape. Duplicate the Shape Layer and flip the x size (-100%).
Position the flipped layer.
Precomp both and use as Track Matte on your footage.
[Andrew Wagstaff]"I am trying to create the effect of a doorway opening in the sky... is it possible to make one mask then copy and rotate it so it will be the same on the other side?"
You may be asking for two different things. If you really want to create the illusion of a door animating open in the sky, try this:
In a nested comp, add the sky layer and duplicate it. Make them both 3D. Change the a position of the upper layer to -0.5, and the lower layer to +0.5.
Create a rectangular mask in the precise dimension AND location of the closed door. Once you have it, copy this mask and paste it onto the lower layer. Set the lower layer's mask mode to Subtract. You should now see the complete sky.
Move the upper layer's Anchor Point on the x and y axis until it matches one of the corners where the hinge should be, then reposition the layer until the seams become invisible.
Animate the upper layer's y rotation to simulate the opening door.
You will now see the layer open onto a transparent background, and the door layer will appear to be a mirror image of the sky as it rotates open.
Former Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA