This is something that is going to be different between each rider/2-up scenario. You and I might weigh 185lb, but our suspension setups will be drastically different between our geometry, handling preference, etc.
Start with this thread and start with the stock settings on the shock:
Whats yours set at?
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Generally for 2-up riding most just change the Preload adjuster (more preload stiffens the spring up so it won't sag as much with 2 people) because it's easy. But 2-up will effect the rebound characteristics and handling of the bike.
My suggestion...start with the shock adjustments both in the fully closed (hard) positions. Go for a ride with a mixed bag of twisties, potholes, bumps, dips, etc.
Then turn 1 adjustments all the way open and go on the same ride. Next ride open the other adjustment all the way up. Now you know what they all feel like and what adjusting them does. Then you can start dialing in the preload and rebound for yourself and 2 up. The preload essentially sets the sag of the bike...static weight. This will directly effect the bottom out of the rear shock. You want to use the MOST travel as possible without bottoming out. That is your preload adjustment. Rebound is really a personal preference based on rider weight, skill, riding conditions, body position, etc.