I like the simplicity of the Viking semi-independent drive system, but it's sort of the reverse of what my wife and I would probably need.
I'll be the driver because I'm certain she wouldn't want that task.
I don't mind pedaling if she is, but she needs to be able to rest/stop pedaling whenever she likes.

So we'd have a few options:

1) Steering/driver in back (more complicated steering)
2) Stoker freewheels (semi-independent, reverse of plans, more complicated chain runs)
3) Fully independent drive (jackshaft required)

Fully independent could be achieved with either two derailleurs on a common jackshaft, or two single freewheels on a common shaft with one derailleur behind that.

To put two freewheels on a single shaft, you'd actually need a shaft that comes apart in the middle - maybe it could use a 1/2 inch or larger square ratchet extension welded in the middle.
I think 4 pillow block bearings would be required for the jackshaft as well - to support the two halves with two bearings each.

Is there an easier way to accomplish an independent drive? If not, I think this is not much more complicated than the streetfigher rear end.