I had that option in my 2009 Mondeo, and it used a "Mono" switch.
To be honest, it's only temperature that has dual controls in the V70, and those controls are so close together that the effort in operating them separately is minimal.
The cost of introducing a mono switch just to save having to move one's hand to the other control instead of pressing the mono switch was probably viewed as not worth it by Volvo since it involved additional complexity and cost for minimal benefit.
I missed the feature for two seconds when I exchanged my Mondeo for my V70.
