i would say u can't(if b.jsp is not included through a.jsp). Couse 2 different jsp r nothing else then two different classes. And on class have no ideea of the methods of onother. Think about takeing the method logic out in some other non-servlet-helper class/been and the instantiate the class from the jsp and call the method.
If u need just some data, use setAttribute("whatever") in apropriate scope a.jsp and call it via getAttribute("whatever")