FCP: Two Way Mask in One Step?

Image Mask works with video... However, sometimes it depends on where the source is. If it's in the storyline it can end up just being a still frame. If that happens you have to use the clip in the Event Browser. You can duplicate a clip there, Open Clip in timeline apply needed effects*. Return to the storyline and apply the duplicated clip into the drop zone of Image Mask. (If you don't need to add effects, you don't need to duplicate the clip.)

*For this kind of effect, I usually use Threshold applied to clips before dropping them into the Image Mask effect. If I'm working with green screen, I'll use a coloring effect and select the green (with Effects Mask > Color Mask) and color it black (or white), then use a second instance of the coloring effect, select the black and Invert the mask and color it white. (Inverting the arrangement if needed, is simple.) Naming the effect is not allowed here (as it is mine and commercial.) You're artwork is already Black/White, so this is just info for what I show below.

This uses Luis' general suggestion for using Image Mask (and as you can see, it's a different frame within the video clip used as a mask):

but uses the Luminance Source Channel of Image Mask and does not require using the Compositing Blend Modes. [My version of Image Mask does not have an Option for a Black/White Source.]

Tried to update my previous post, and it wouldn't let me, ignore it, I was on the wrong path there. Here's how to do it:

You'll put the video of your shape into a compound clip with the background you want to fill the shape itself.

Shape on top, set it to the "Stencil Luma" Blend Mode.

This will fill a black shape with that video as one clip.

Repeat for your second b/w shape video.

Stack on top of each other.

The main background you want for the full composition should be in the Primary Storyline, with your two compounds on top.

I would use the black and white clip as an image mask, it should be very easy.

1) Place clip A atop clip B.

2) Apply Image Mask to clip A, and select the black and white as the Mask Source.

Select Luminance as the channel, and check "Invert Mask". Done!

Ok, good point.

Here is an alternative that works with video and does not require you to use the mask clip twice.

1) Stack the three clips:

Mask

A

B

2) Select the Mask clip and set the Blend Mode to Silhouette Luma.

3) Select Mask and A and turn them into a compound clip.

You're correct, shape mask won't do it here. What you want to do is change the Blend Mode of each copy of the clip. One to "Stencil Luma" and one to "Silhouette Luma."

But you still need to put each copy, along with it's background, in their own, individual Compound Clips.

I've done what you're trying to do several times in the past, it's a really neat trick.