Most banana seats can support two people, but most sissybars cannot. The over the axle type tend to bend to one side or the other when overloaded, and will eventually collapse. The other syle which is clamped to the rear dropouts with a sliding clamp work pretty well. I have used them on several bikes and I am 220. They will slip, but the tubing is usually a thicker walled type that is stiffer. If you can get the clamp to hold both of you, then yes, you can ride it double. I would not trust the axle mounted type for more than one person, they just don't have the strength.,,,,BD
So many bikes, so little dime.