Hi,
I am curious as to what you mean by dual directional coupler. Are you
referring to hardware such as the devices available at:
http://www.e-meca.com/rf-directional-coupler/dual-directional-coupler-722N.htm
I need to know which Natioanl Instruments product you are asking about in order to be able to answer your question.
Best Regards,
Evan R.
Applications Engineer
National Instruments
http://www.ni.com/support