This may be an over simplified question but why not just eliminate both superchargers and place a 8.3 unit on the same engine modify the pulley system and get 8.3 litres of sc power in a single unit.
Easier tuning and management etc...
The 2.9 plus 2.9 do not exceed the volume of your 8.3 unit. The only reason I can think of is pulley changes would be too difficult and customer wanting to retain the dual sc aspect of the engine ie...for re-sale.
Can you elaborate on the rationale.