Up the rear preload (i got mine one notch from full (hardness) i.e. position 6 of 7) and just account for the extra weight.
It dosnt really make too much differance to the handling (not that I can really tell, but my shock is tired) but its probably the sensible thing to do. I've done loadsa 2-up miles, including 420 miles at the recent annual rideout weekend. If anything, having Maria pillion made the bike more stable, specialy over bumps and in the high winds. Also, she kept me warm in the rain
Riding alone is great fun, but being able to share the experiance with someone riding pillion wth you is incredible. Just remember you are responsible for your pillions enjoyment, and work out some sort of signal system so they can tell you when they need to get off, when they want you to slow down, when they want to you hurry up and get a move on
Matt