When you need to perform the same deploy in more than one machine at the same time, you can use the HOSTS parameter as follows:

Setup the machines

Before you can use any of the Capistrano deployment tasks with your project, you will need to make sure all of your servers have been prepared, for that run:

cap HOSTS="machine1.domain, machine2.domain" deploy:setup

Deploy

The rest of the task must run same as a normal deploy, but always using the HOSTS parameter

cap HOSTS="machine1.domain, machine2.domain" deploy

Multistage deploy

cap HOSTS="machine1.domain, machine2.domain" stage_name deploy