I would think the best way would be to get a 8 inch inline fan and run air through both 8's through a 8 to 6 reducer and through 6 and out to wherever you're exhausting
Unless you can afford a wye. Then I'd say hook the 8 inch fan up to a wye that goes from 8" branched into an 8" and a 6" with the 6 inch hood on its own line from the wye and the 2 8 inchers on their own line. That would be the absolute best way