hi,
the best it's to have a triple INS configuration!
Why?
Because our software,being adherent to the realty, can send you randomly the INS's failures and in case of an INS failure where's expected to shut down the INS the navigation can be continued having two INSs.
Why it's better to have two INSs than one ?
Because you'll be able to use  the INS#1 for the current navigation and the INS#2 for the DME updatings.
To perform a DME updating you have to tune a DME station at least 15NM off track and it's easy to understand that a station 15NM off track it's not useful for  the current navigation.
Moreover if the software sends you a fatal failure you'll remain with a single INS configuration.
Finally:
for extended oversea flights you should  have a triple configuration also for the possibility to fly in triple mixing,
For short&middle range flights(from 1hour to three hours) it's acceptable to have a dual configuration.
Single configuration it's reserved for short flights made with private single engine planes like Cessna or Piper.
Regards Gianfranco