The short answer:
The bold answer:
Hide the second pi somewhere, it's small enough.
The expensive answer:
Combine the two videos to one, and use a HDMI split-screen machine to separate them again.
The hand-soldered answer:
Combine the two videos, so that one video occupies the odd lines on the CVBS, the other one the even ones. Then split the CVBS using a video amplifier, and build some extra electronics which blanks out all video lines (not the sync) every other half-frame on each output, respectively.
The sensible answer:
If the second screen has a low resolution, like 120x90 or so, it might be possible to use a SPI connection to display a video. This size of video would also be playable without hardware decoding. Using a USB-VGA adapter (thanks for the hint, I almost forgot that
) even 320x240 or so might be possible.