True Duplex? Where both people can talk at the same time and be heard simultaneously at the other end? The only way I can think of is two radios, one for each frequency - unless you've got lots of money for a WinRadio G39DDCe where you can listen to two frequencies at once.
I think you're referring to 'full duplex' which is not taking place with marine radios AFAIK. What actually takes place is usually referred to as 'half-duplex' (each person transmits on a different freq, but no one can hear while transmitting). In older books, I can remember it being referred to as 'two frequency simplex.'
Here in Virginia, I don't know of any duplex except SeaTows automated radio checks on Ch-20 in Norfolk.
Most of the older 'public correspondence' channel pairs have been leased here to the Virginia State Police for use in its VHF P25 trunked system inland. I usually have to lock out the 'B' channels to avoid hearing STARS when I'm scanning marine.
As Bill described, you can listen to them by scanning both freqs (or the whole band including them) with your delay turned off. Works fine. I do it all the time in other applications.
73/Allen (N4JRI)