Here's an AE quick tip (COW video tutorial) that might help you. It works with transparency, not luminance, but it does let you use more than one layer for your transparency control.
If you want more info, or can't watch the video,
here's the AE help page on the feature Aharon is discussing.
- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')
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