I've seen a similar setup on a koi pond. The Ultima is better with one pump because it cannot handle that much flow/pressure. I've seen the inside laterals broken letting the media out into the pond. With two pumps and a check valve, the stronger pump will try to close the check valve on the weaker causing it to stall. This is because of the back pressure from the filter and uneven head loss on the two pumps. If you decide to use two pumps you should use clear check valves so you can balance it with the ball valve.