Suspension
You do need to adjust preload for 2 up or solo riding if you need the best handling from the bike.
You youself must figure out what's the best setting for spring preload (the big knob on the l/h side of the rear shock you can adjust yourself and has a scale on it too (look closely)) for both settings (it'll depend how much weight you and your pillon are) - read the setting from the scale on the knob and remember them - it's easier next time to switch between solo/2-up riding. Also on travels, if you carry lot of luggage, it always needs more preload.
And the other thing that needs adjusting along with preload is the dampening. Dampening is the small screw on the bottom of the rear shock.
Turn it fully to minium (counter-clockwise) and then clockwise to measure how many turns it is from minimum to maximum. I.e. mine is 3.5 full turns from min to max (preload and dampening and the connection between them).
Dampening mostly always goes hand-by-hand with preload. I.e. if you put preload to 1/2 of the max, then i need to turn dampening screw around 1.5-2 turns (also 1/2 from the dampening screw scale).
The logic behind it is the design of the suspension. Spring acts as a elastic element, shock acts as dampener for spring. Spring alone wouldn't work as a suspension since if you go over the bump it'll shake up-down some time till it stops (after it's exited it takes time to dampen by itself, like a guitar's string). Shock alone wouldn't work, since it'll just collapse under bike's weight w/o spring. Thus if you make spring stronger (adjust for more preload), it needs more dampening to stop it from "shaking" (you have to adjust dampening screw accordingly) after going over a bump or entering/exiting a sharp curve (adjusting dampening is just like you put your finger on a sounding guitar string to stop it in the way you like it, and also it affects how it starts the string). Too much dampening will make the rear overdamped (it'll kick hard going over bumps), too little dampening doesn't stop the shock from shaking by it's own elastic forces. So you must find your own balance between comfort and ridability of your bike.
Anyways, the best way is to test different settings, try your bike in different terrains and riding styles, and you'll get the "feel" how to adjust suspension.
