Zakeen:
I would think for you a 6 hour ride would be easy. Why worry about 2x3 hours???
Mentally, getting your head into two rides a day can be a lot, even for riders who are highly motivated by lots of volume.
I've found that if I'm riding well towards the end of a long ride (5-8 hours), my power increases as each hour passes, something I don't think would necessarily happen with 2x3 hours.
There is something special about the last hour(s) of a very long ride when your form is good. Flying on the flats seems easier, climbing seems effortless, it's weird but true. Maybe we really need 3-4 hours of "warmup" before we can really jam. I remember former Dutch pro Erik Breukink warmed up over 4 hours for a prologue of only a few minutes in the 1991 TDF.
Longer rides also force you to feed and hydrate yourself, something 2x3 hours would not do in the same way, at least not on the bike. Additionally, your back, neck, shoulders, arms, etc. will accumulate additional fatigue on a continuous ride. These muscles have to be trained through long continuous rides in order to be able to handle long, continuous road races without discomfort.
If you have 6 free hours to ride, by all means take advantage of it while you can!!!
Happy big miles!!!