Flow depends a lot upon the area of the tank, what your aquascape is like, and what kind of livestock you plan to have. The nice thing about a Biocube is that it doesn't take a ton of hardware to get water moving since the width and length are pretty small.
Most folks who point their pumps at one another are trying to create a more chaotic flow pattern in the tank. When water is pushed straight out from the pump, (laminar flow), the force can aggravate some types of corals or cause the sand to shift. By combining flows from multiple sources, the flow becomes more chaotic and turbulent, which is less intense within the tank and creates more flow all over, reducing the likelihood of dead spots. Dead spots in the tank are where food and detritus end up rotting away, increasing nitrate and whatnot. More chaotic flow also improves coral health by giving filter feeders more feeding opportunities.
Some people prefer to go with a multiplier approach when selecting circulation pumps. Like 10-20 times the volume if the system. I have found that it is more about placement of the pump than it is about total GPH.
For the most part, despite what folks will say, the best thing to do is move your pumps around and see what gives you the best "all over" flow within the tank. Look for places where stuff circles around to identify dead spots. They look like under water dust devils, if that makes sense. Also, check to make sure you are getting flow all around your rockwork, particularly behind the rocks, to ensure that you don't get food and critters decomposing behind the rocks where you can't clean. Also look at any corals you have and see if they are retracting polyps. Sometimes you will see retraction on one side and extension on the other. That's a good indication of flow being to strong from one direction.
I usually don't count my return pump in my overall flow assessment. However, from what it sounds like, you probably have plenty of flow for most corals and fish. Some SPS, like acros, really like strong flow, so if you want an SPS dominant tank, another 425 wouldn't be a bad idea.
HTH
