I wonder why for fastest human-powered they don't use more than one cyclist. I assume with eight humans like in rowing you could get up to a ridiculous speed. Might be a little dangerous though.
I'm curious what multi-cyclist vehicles could do, now that you suggest it. With the lightness of these vehicles, at what point would you have to add spoilers?
Sadly, the video is pretty cheesy and content-free. It's hard to recommend.
However, the speed would nowhere near double. Power required to overcome drag force is related to velocity cubed. So if you double power, the velocity will increase by a factor of (2)^1/3 or the cube root of 2, 1.259.
So if they added another rider of equal power and managed to keep drag the same (it would actually increase a little) they'd go around 25% faster or 107 mph. You may run into other problems like stability and bumpiness first though.