Hey, that's a neat accessory!
Popo, I had the same problem when I was viewing the space shuttle. I only had one major tripod and wanted to use more than one camera on it. I ran out of time, so just mounted my camcorder on the hotshoe of a 7D which was on the main mount. Worked fine for a light piggyback so to speak, but I'd have preferred a side-by-side plate.
They call them side-by-side plates in the astro world don't they?
Like the Losmandy VSBS near the bottom of this page...