I see no reason for the 6500 pump?If it is in series behind the 10,000 pump?The 10,000 will still only pump 10,000 regardless of if the 6500 is there or not?
I can see a reason why I would try it , the total output will be 10,000 regardless OFC, But as long as a bypass is fitted so the water the 6500 doesnt take can return to the tank unimpeded it should work, be better is on a separate return inlet OFC, plus if a bypass is fitted it technically isnt in true series its more of a spur. Plus it means he can run the CO2 reactors at a higher pressure which should increase CO2 uptake by the water . Plus using the feed from the 10,000 means less piping which is good IMO. My independent lines gets dirty and cant wait to get rid if it
Then boost the tank turnover with a couple of gyres, no piping easy to clean great flow
Just my opinion