I don't think that there is any one answer for that question and our machines that were not designed for two people.
A seat that is great for kids won't work for adults and so forth.
I ride two on my Kodiak quite often and all I have is a old ITP canvas bag with a cushion. It has the storage pockets on the sides and the back and works, not as good as some but better than others. You just need to decided what is best for you. Usually when I have a rider he is 230 lbs so some of the seat type won't work.
One thing you are going to have to deal with if you do ride two on your machine is the heat that comes off of the muffler and out of the air ports on the left hand side. It gets hot enough that the passengers left leg is going to be very uncomfortable if not just down right miserable. I purchased some heat reflective tape in a sheet and installed it on the underside of the fender to reflect the heat back downward. It isn't a complete fix but it works.