Hey Zak, because we run them lower power than max I guess you could call it over kill. You could probably do the same with 2 20s or a mix, the catch I thought was only the 40s had the wireless controller to run master/slave, that may have changed now, I'm not sure.
The way I look at it though is bi-directional flow. If you run your single on the same wall the flow even with reef crest is always coming from the same direction. By getting another pump on the oppisite wall the flow truly shifts directions. Then when you get into master/slave & Anti-sync, you get the reef crest random high & low cycles along with the flow pulse cycles changing from both directions and you have undertow from both sides also. The tank sees flow from every where randomly, to me that is as close as we can get to the ocean.
Have fun with your tank!