Ventisit is a onepiece , so it replaces both pads. You should e-mail them and explain you have an older Sofrider/Freerider. The website looks like it only has S40 and V20 sizes.
For what it's worth, here is my experience with the Ventist on my QX100.
Not quite enough 'bottom' cushioning for my liking.
I removed the the seat pads from the original seat and used velcro to attach the dense foam pad to the seat pan and more velcro to attach the Ventist to this foam pad.
Works like a champ for me.
Did my first century several weeks ago and all was "comfy" for the entire day!