1. Create three video tracks.
2. Put your background event on track 3 (bottom-most).
3. Put the event you want to be 3d in track 2.
4. Click the track motion button for Track 2, and select 3D Source alpha. Adjust size, rotation, etc. to taste.
5. Make track 2 a child of track 1.
6. Now here's the trick: in track 1, go into Parent Motion (NOT Track Motion!) which is a button over to the left, just under the track number. Select 2D shadow or glow, adjust to taste.
Do all your 3D motion stuff in track 2. Track 1 just acts as a container for the track and allows you to add shadow and glow.
Thank you for the solution. Unfortunately my Hitfilm2 Ultimate has some problems with png files (media offline error). I'll try to contact Hitfilm support and then let you know if it worked.