separate log info from a particular method

Hi, 

I am using RollingFileAppender for my whole application. Suppose the
main logfile is A.txt. I want to log all information from a particular
method to a separate file say B.txt (these log should not be logged to
A.txt). StringMatchFilter filter should be work, however the
particular method has to log a particular string in order to use the
StringMatchFilter fileter. Is there any other convenient way to
archieve this? Ideally just add some config settings.
thanks