For best results you should get the right spring for solo riding and increase the preload for each two up ride.
If you get a spring made for two - it'll be way too stiff for solo.
But then again, if you get a solo spring that's a little bit on the stiff side - you'll most probably be fine riding with a medium sized passenger.
(not perfect, but fine)
exactly, we have to crank the spring now, if i got a better spring for me where the preload rests somewhere in the middle setting, we could up it for two. I like that idea.