Construction hints for a sociable dicycle?
Some thoughts - but not really an answer.

Fundamentally its a RWD tricycle with no front wheel.

Drive  shaft drive.
I'd be exploring a driveshaft that runs horizontally under both seats, and using a tricycle-style open differential in the middle.

There is a shorter jackstaff drive axle that runs from the rear diff up/forward to the half-shafts through a closed differential. Otherwise this would not be rideable solo.

Drive  chain drive.
For this you'd have a chain from a front pedal axle to the rear axle.  This would allow conventional bike components to be used for chainrings and rear cassette, and even deraileur gearing.  However you'd want to incorporate two more freewheels out by the wheels themselves, to allow for skid steering

In this case both riders will pedal at the same cadence, and the spare cranks will rotate when riding solo.

Some excellent concepts of "chain drive to two wheels" show up in this answer https://bicycles.stackexchange.com/a/38875/19705

Final drive 
To get drive up from the under-seat cross axle up to the wheel hub, you're probably looking at another chain on each side.  Good thing about this is the gearing could be swapped out as required.  

Steering
Steering would be achieved by skid-steer, braking the inside wheel on the turn so the outside wheel drives around.  Ideal turning circle would be twice the horizontal wheelbase or width.  Brakes would be best as disks attached to the wheel hub, outside of all the chains.  That way any drive failure will not stop you from braking.  

Brakes
I'd contemplate a pair of motorcycle-sized disk brakes, one per side.  Possibly use two bicycle calipers per disk, and connect each pair to each seating position.  That will give a redundant braking solution.

Visibility 
Too much framework and it will be difficult to see around.

Roll-around 
Gerbilling will always be an issue, so put in a pair of three point racecar harnesses in case you end up inverted.   Likewise, racecar seats would be appropriate rather than bike saddles.

Off road gerbilling would be awfully dangerous - even flat-land there could still be an obstruction that catches part of the frame, or part of a rider.

Safety
You might want to consider drag rails at the back in case of "wheelie" when taking off, and possibly a front outrigger too (Though this would need a wheel or a skid/ski to prevent digging in)

You require wheels to be larger than the maximum "size" of any rider so that no part of the riders comes outside the "can" shape.

Other
The passenger's hands will be bored.  Consider adding a cupholder.

Side slope - most roads have a degree of slope to assist water running off.  Even pathways and sportsball courts are generally not completely flat.  So your ride will be canted in one or the other directions almost all the time.  Whether its worth allowing the seats to rotate or not may be over-engineering.