Can you shift the topbox forward, to sit where the pillion seat goes? I seem to remember it being possible on earlier versions of the awesome 1200 steed.
Quite why you'd want a Ventura system (designed to replace hard boxes) and a topbox is a mystery.
On a bike I once owned with no panniers, I had a Givi E55 top box and put a Ventura Spada bag on the passenger seat........... Not sure if this is a option.