From the guy who taught IGS for a few years at NUW, we put it out (and this was based on the AF's uber super duper IGS course in TX) that both pilots needed to be able to see the PRIMARY flight instruments. In the Prowler both the pilot and E1 could see the EFIS, E1 had an "instrument rating" and the crew could shoot approaches under the umbrella of being dual piloted. The Hornet (deltas) or rhinos (foxtrots) are not considered dual piloted because the primary flight instrument is the HUD and the backseater can not see it - and yes I know you can pull various things up on your MFDs in the back but apparently it doesn't count.
definition per the 3710T & U...
Single-Piloted Aircraft. Any aircraft that has only one set of flight controls or a tandem cockpit, or any aircraft that has two sets of flight controls and instruments and is being operated by only one pilot who meets the requirements of the NATOPS manual for that model aircraft.
with that definition, unless I'm f'd up... any tandem seat aircraft is considered single piloted. Any side by side seated aircraft is single piloted if it doesn't have two sets of controls & two NATOPS qualed pilots.
Not sure why tandem cockpit aircraft with two sets of controls & instruments with two NATOPS qualed pilots is considered single piloted... it can't be due to aft vis for the backseat pilot because, as Gator brought up, how would the rules work with an IP in the trunk of a T-34/6/45 with a student in the front. The student isn't NATOPS qualed. My guess it has to do with crew coordination with side by side seating vs tandem seating.
SF