I am not sure about that, I mean I have have had thing singed by more them one person at the same time but I e-mailed one of the people and ask if the they have everyone sign it.
I think you could just send it to one person and put in a letter and ask them if they can have the other person sign it to.
Or if you want just address to both, I really dont think it matters.
Duct tape is like the force. It has a light side, a dark side, and it holds the world together."