A Varios 8 can handle a little over 7 ft of backpressure at 1500 gph which is reasonable with the proper sized pipe. I don't think a single 1" full siphon will handle 1500 gph assuming it is a typical aquarium on a stand with the sump directly under the tank (4 ft of elevation difference). This will not give you a full back-up overflow. I generally would allow only 300-400 gph for a 3/4" return line. If it is only the final bulkhead connection it can be higher but I think getting 1500 gph with two of them would be a stretch so I still think you are going to be limited. I would run separate 1-1/4" return lines to each 3/4" connection.
For 1500 gph I would want 1-1/2" drains and two 1" returns, but I doubt you need or even want that much flow
I am not sure how big your tank is, but unless it is really large (over 265 gallons), I would use the two 1" drains in a Herbie and run two 1" lines to the 3/4" return bulkheads and reduce down with a reducer bushing at the bulkhead. This should give you 600-900 gph with a Varios 6 pump at full speed. You may want to use a Varios 8 to run at lower speed. Most people on this site seriously overestimate how much flow they are actually getting. I think 3X turnover is more than adequate on most systems and I think you would find the turbulence of 1500 gph on a typical sump unacceptable. Think of it as enough flow to fill a 5 gallon bucket in 12 seconds.
