Exporting a separate Alpha layer from AFX and using it in FCP?

Hi Everyone, thanks in advance for your help with my question..

I'm working in an HD (DVCPRO HD 720p60) project, and we have graphics that are being created in After Effects for our project. Since they are about 10 minutes long, and need to be composit-ed over the material in our timeline, we'd like to export them from After Effects in the same compression / resolution settings as our sequence. However, when we choose DVCPRO HD 720p60 as an export setting in After Effects, it won't let us render the final QT with an alpha.

SO, we have rendered two files, 1. the Animation and 2. the Alpha channel (only). Both were rendered at the sequence setting, and were brought into FCP - NOW, we are having trouble finding the effect which will enable this "layered" alpha system to work. We have the Alpha layer under the graphics layer, and tried all the different Key and Matte effects in various combinations, to no avail.

Has anyone ever done this in Final Cut Pro? In other words, instead of relying on a built in alpha channel in your graphics, has anyone ever done it the "old fashioned" way by using layers (a layer with your animation, and a separate one with just the alpha channel)? And, if so, which effect did you use within Final Cut to make this work?

Thanks again for your help!
- Jennifer