Well, it appears that no one here has experience with Thule or OEM racks on a Volvo S60.

The aerodynamic cross section profile of a mounted tandem on your Topper won't be any different (i.e. it isn't going to get longer) therefore if the Topper is securely attached to the proper crossbars (specifically designed for your S60 roof) and the crossbars are securely anchored then you shouldn't have a problem with crosswinds even if the spread between the crossbars is shorter than what you were used to on the wagon.

I checked the Thule site for the S60 rack but it doesn't appear to mention the actual spread between the front and rear crossbars. How much shorter than 31.5" is it?