vfx Double Chroma Key tutorial

While making my bands new music video, I couldnt figure out how to create a double chroma key shot/effect -where both a green and blue screen can be keyed out in the same scene. Trawled the internet for a solution without luck. After a bit of trial and error, figured it out myself and would like to share with the interweb.
If you have a scene that contains both blue and green screen, where you would like different footage appearing below the blue and green elements, here's how to do that (in Vegas specifically but the concept would work for other software too):
  1. chroma key out the blue on the track, have desired background beneath it, select "Cut" composite mode.
  2. Render as 3mps Quicktime, prog scan, PNG format, 32bpp color.
  3. Import file back into project. Select properties, Media, Alpha Channel, premultiplied (or Straight).
  4. Ensure blue chroma key is now off & composite mode is set to source alpha. This track must sit higher in track order and be 'above' the green elements below.
  5. Now chroma key out the green on the track, have the desired background beneath it.
  6. Done!
Here is the result. As the hand swipes the ipad, it's actually swiping from a blue to a green to a blue etc picture.

I don't know Vegas (I primarily use Fusion or Nuke for compositing) so I don't know if this would be even possible for you.
If I was to attempt this I would try and achieve it with multiple input nodes (or layers) that are then composited within the software. As a rule of thumb in VFX you should limit, unless really necessary, any rendering, and re-importing, as there will always be, regardless of the compression, some degradation in the source material.
That's just my 'tuppence' worth, because, generally, if your final destination is (like your clip) youtube, you'll never notice the loss in quality because of the further compression that youtube,themselves, slap on it.
Great video by the way.
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