You can link one mask path to another using an expression.
If the mask that you want to use as the master is on layer 1, then just make a dummy mask on layer 2, add an expression to the dummy mask's Mask Path property, and then use the
expression pick whip for that expression to link it to the Mask Path property of the master mask. (I.e., drag the pick whip from the dummy mask's Mask Path property to the master mask's Mask Path property.)
Then you can invert the "copy", change its
mask mode, or otherwise customize properties of the mask.
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Todd Kopriva, Adobe Systems Incorporated
putting the 'T' back in 'RTFM' :
After Effects Help on the Web
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