How can I enable access to CircleCI 2.0?
CircleCI is available for all customers. Simply create a 2.0 config file and your build will run on CircleCI 2.0: https://circleci.com/docs/2.0/hello-world/
Can I still use version 1.0 for some of our projects and 2.0 for others?
Yes! You can even use 1.0 and 2.0 on the same project :-)
Is it possible to have git branches running with different CircleCI versions?
Yes - see the link above.