I did a project in Motion 1 (M1) in which I added a round mask to an object a coin), and flipped it over revealing the back. I did it by keeping Y axis the same and adjusting the X.
Anyhow, I attempted to do this with the 3D filter in M2 and cannot get the back side to be frouped with the mask. It goes into the well, okay, but the full picture keeps showing up on the backside, rather than the masked version.
Try putting the object to go in the backside in it's own layer and drop the layer in the well. You may need to put the mask on the layer instead of the object. I've been working a lot with that filter and I have to say I don't like it very much - makes text quite fuzzy and it's very fussy overall.
Apple-certified instructor, FCP, DVDSP and Motion
Author, Motion Visual Quickstart Guide from Peachpit Press http://www.applemotion.net