Thanks for the input guys, the comment on the state of the automatic systems is one aspect I hadn't thought of. I wasn't originally going with the angle of whether 2 pilots is better, of course it is wonderful to have an extra set of eyes, hands etc. My mrs, while not a pilot is quite adept at looking after charts and generally keeping an eye on me and keeping me on the straight and narrow, spotting traffic, writing down clearances, tuning radios navaids etc.
I've only had an hr in the right seat of a new Citation Mustang and it does seem the workload is way lower than an old Baron/310/Seminole with steam gauges, even with the Mustang being twice as fast. The new Garmin kit is bloody wonderful. Partners who fly often with their spouse should have enough training to get an aircraft on the ground should something medically happen to the PIC.
Comments about weather etc are moot, while I'm qualified and experienced IFR, when flying for personal reasons, if the weather is hard IFR, we stay another day by the pool or in the pub. That's the best part of private flying, you don't have to go when the weather is bad. This seems to be quite common with the Mustang owners I have spoken to.