I'm not sure if i understand you corrently, but calling a Class method in Template would go like this:
{{ MyConroller->doSomething() }}
Of cause you could call one controller within another too... just use raw php
$obj = new MyController();
$obj->foo();
or use the F3 call method $f3->call('MyController->doSomething');
Also check out the new API docs.
http://fatfreeframework.com/base#call
It's still under construction, but hopefully you'll find more information about this or any other framework part very soon.