It sounds like Deepak s RSpec Multi Mock gem solves this.
But you should ask yourself if this is really what you want. Ultimately, consistency is most important, and using a mix of mocking frameworks within the same app is bound to lead to confusion.
Personally I m quite happy with RSpec s built-in mocking and don t usually find the need for Mocha on my own projects. But some of my peers prefer Mocha and I m not really aware of any limitations of Mocha compared to vanilla RSpec mocking, so I d recommend sticking with Mocha if that s what the app already uses. Even though I know its "any_instance" support is considered a code smell by some, it does occasionally come in very handy. :)