Yeah now that I think about it the guy I brought the setup from had a whole garage full of those 100g rubber made containers he was harvesting liverock in. It was all disasembled when I bought the tank, but I bet he had 8 of those plus the tank. I only planned to use 1 of those hammerheads on that tank. The tank has 3 2" drains in the bottom of it then it returns to 8 1" down spouts mounted in the top of the tank. The refugium alone was 100g. I should have passed on the setup as the tank is scratched to hell. I was going to buy a buffer sand it all down them clean it up one day. Once I got it to the house I realized it would not fit anywhere so its on the bakc burned until we build a house in a few years. Its going to be a room divider then. I hope it turns out nice. If not I will have a long coat closet!
I think one Hammerhead is reasonable, especially necked down to 1" returns. If you check out VLDesign's tank, he's running two Hammerheads on a 2600 gallon tank. When you wrote two Hammerheads on a 350 g tank, I was picturing major whirlpool action. We have one 1" spa filter return nozzle on the far end of the tank. The current travels eight feet across the tank, hits the far wall and starts heading back the other direction. It eddies near the middle of the tank. Not all fish like the strong current. Our big plecos hated it and spent most of their time on the other side of the overflow box farthest away from the return.
I am in no way putting down Hammerhead pumps. I love Reeflo pumps and wouldn't consider any other pumps. Our current Hammerhead has performed flawlessly for nearly 6 years. I bought a backup Hammerhead five years ago, and it's still in its box.