I'm not sure exactly how subaru accomplishes their AWD system, but with everything else I know of the VC is in the transfer case. The Jeep quadra-trac AWD system(New Venture/New Process 249 Tcase), and the Jeep "Select-Trac"(New Venture/New Process 242 Tcase) are done this way. The difference between them is that the latter also has options for 2wd and traditional part time 4wd(high and low range). From the point of the Tcase, power transfer/delivery to each wheel is determined by the type of differential used. Locking differentials or spools are a no go on AWD systems, so that leaves either open or limited slip types.
Also, viscous couplers are used because they allow the front and rear wheels to rotate at different speeds. This eliminates the binding found in traditional 4wd systems because the front and rear are not mechanically locked together by gears, or the drive chain found in modern transfer cases. That said, when the viscous coupler fails, the difference in vehicle handling will be noticeable, behaving more like a traditional 4wd. As I understand it, the Subaru's are prone to VC failure, as are the Quadra-Trac systems. From what I can tell Subaru's "symmetrical" system is nothing more than marketing hype based on equal length axle shafts.