Probably you have guessed it, you need 2 phones to accomplish the effect.
Considering the
impact on audience, the idea is not bad. But I don't think it's an ideal 1-man effect. If you love
good mentalism and have a helper (your spouse, partner or whoever), then this effect will surely
work.
By the way, the production quality of this video is really bad and it is one of the
worst home-made video I've ever seen. And the idea of doing this in stand-up situation is just
absurd.
At US$8 dollars, it is hugely overpriced. But I think it will be fun to do as a
2-man effect.