Jeffrey is correct, the Marketo sandbox is like having a second instance (but must have less traffic) with a diferent munchkin code account.
With enterprise you get a sandbox. The issue I have is whatever you build in your sandbox, pretty much needs to be rebuilt in your production. There is no push to production. With the exception of the Program and templates import library. We have a developer website that the sandbox munckin code is on and only internal network visits are collected.
We are currently have 2 instances using one domain. We decided to split out into Corp and consumer businesses.
Corp with all the bells and whistles, and Conusmer with less toys. Both munchkin cookies are placed on there business areas of the site and each business unit have there own forms.