I have two controllers a and b.
I would like to call a method of controller a from a method of controller b.
I posted a somewhat similar question a while back, but regarding a model on CI.
Returning two separate query results within a model function
Although your question is not exactly the same, I believe the solution follows the same principle: if you're proposing to do what you mention in your question, there may be something wrong in the way you're coding and some refactoring could be in order.
The take home message is that what you're asking is not the way to go when working with MVC.
The best practice is to either use a Model to place reusable functions and call them in a controller that outputs the data through a view -- or even better use helpers or libraries (for functions that may be needed repeatedly).