There have been a few postings that deal with this issue. ITSATDM found that lowering the front end cured it. As did I.
After a complete front end change, to a front that is about 40mm taller, I found that having more, or less ride height in the front seems to stop the instability. The standard ride height in the front seems to be particularly susceptible to ride height changes in the rear.
That's why putting your wife on the back fixed it. It's effectively made the steering angle much lazier than standard. Make the steering angle a bit quicker than standard by dropping the front end and you'll fix it too.
For 2 up fully loaded travel I went to a Ohlins rear, revalved and resprung with a 23kg/mm spring - the heaviest they make.
Solo I have no preload on it and I get about 60mm rider sag. With full preload and wife and gear I get 60mm. No instability.