You can run 9.5" in front and 10" in back w/ AWD as long as the rolling diameter is the same. But there's no point to do it over .5". I run 9" in front and 10.5" in back on AWD 265/40/22 in front and 295/35/22 in back.
With 9.5" and 10" wheels, you'd have to run the same tire size (285/35 is fine) but it would have uneven stretching in that the front would bulge more than the back. It would be silly and you could never rotate your tires. 285/35 is fine on a 10" wheel.
And FWIW, are you sure your guy offered you 9.5" and 10"? I don't think Vossen even makes those sizes. In 22", all they have is 9" and 10.5". You can run those sizes, but you would have to size your tires appropriately. Make sure your guy knows what he's talking about and orders correctly.