With those two pumps you'll have about 4000gph (not counting head loss). I have a feeling that a 1.5" Beananimal system will handle that just fine. Or if that makes you nervous, a 2" Herbie will be good too. All else equal, the Herbie and Beananimal should flow the same amounts with the same diameter piping. The only real difference is the 3rd emergency line on the Beananimal.
My 400g ran a 3400gph pump on a single 1.5" drain line (regular drain, no Herbie or Beananimal), and it handled the flow just fine (pics of the setup in my sig). Also, on my current setup, I have 2000gph going through my 190g on a 1.5" Herbie system. When I take apart the main line that has the brass gate valve on it, I can look inside the valve and see that it's about 90% closed off. There is seriously a tiny opening. And that tiny opening is handling all that flow. I'm confident my 1.5" system can handle a lot more than 2000gph.
Hope that helps.