We have found pretty interesting picture of a recumbent tandem with two front wheels. It is offered by Turner recumbents from Tuscon, Arizona, USA. Here is the explanation from their web site:
“With two riders providing twice the power as an ordinary bike, much higher speeds on a tandem are possible. Safety becomes paramount, especially when a front-tire blowout, or slipping on a sharp turn, can turn a good day into a disaster. But with the T-4-2, the extra front, in-line wheel provides a critical safety backup. A flat will just slow you, not throw you.
The second front wheel provides this safety without compromising speed or agility. That is because both front wheels are connected by the steering bar, so that even the tightest turns at speed are “locked-in the track” for maximum traction and control, similar to a rollerblade skate. Both front wheels have their own brakes, controlled by a single caliper, thereby increasing front braking power by 100%. All told, there are four sets of brakes, with the driver controlling all three wheels, and the stoker rider also controlling the rear for the ultimate in braking safety.
The frame design follows the Turner philosophy of “strength through triangulation,” not the flimsy, flexy designs often found in the other tandem bikes on the market. Whether used for romantic rides in the sunset or power-pedalling across the flats, this bike is built for dependability, durability, and efficiency. As with all other Turner bikes, the tandem follows the tradition of providing maximum comfort to the riders. The T-4-2 has firm padding on an ergonomically designed composite seat to provide maximum logitudinal power to the pedals along with a luxurious ride. The T-4-2 is sold by custom order only.”