Roll stiffness not only controls the amount of lean of the car, but if you look at the ratio of the f/r roll stiffness (roll couple) then it also serves as the easiest way to adjust the steady state breakaway characteristics of the vehicle. ie: over/under-steer during the middle of the corner.
The easiest way to test this is a skidpad test (or driving in a circle) and slowly increasing speed until one end starts to break away. If the rear goes first then lower rear stiffness or increase front stiffness. If the front goes first then Lower Front stiffness or increase rear stiffness. Repeat until you evenly slide out as you approach the grip limit of the corner radius.
Yes, you can offset different sides because the total twisting force is what is acting in either direction. I believe it would be the equivalent of moving both sides half as much but I'd have to calculate that. If you want to get really into the weeds here is a great thread that Whiteline contributed to:
https://forums.nasioc.com/forums/showthread.php?t=218971