With plenty of ball valves, you can make it work. Like i said. I have a total of 2200 gallons per hour and I still had to put koralias in. I hate them and I wish it was not that way but... ?? Maybe don't get a mag 9 but something smaller. if you split the return, you're splitting the power, so basically if you get a 9, you'd get 5.5 on each side. plus you are right, when it is going 4 ft vertical, you will lose some of that GPH. I would say go with a 6, then split it, and you should be just fine.