Well, I found an answer.
Another stackoverflow mate did the same question here: Use capifony to deploy to multiple production servers
And they gave him the answer, linking this other question: Deploying a Rails App to Multiple Servers using Capistrano - Best Practices
I hope I helped you, mate ;)
Edit:
I didn't realize that you told me "not at the same time". But, if I'm not wrong, could be another solution:
Forget about I told you, get the deploy.rb as standard as possible. Then, if you delete de line where the host is specified (and also user and pass), each time you deploy with capifony you'll be asked to enter the host (and then the user, and then the pass)
Maybe isn't the cleanest solution, but I think it could work...