Go with the flow
What size tank do you have? depending on the size of your tank you'll need to make sure you get enough flow from each powerhead. Also, though I dont have one myself, I've heard that wavemakers are pretty worth while. Especially for corals like nick had mentioned, certain corals need random/alternating curents. I have a 46 gal bow. I have two koralia evo pumps (right now at least) each one is pumping 650 gals per hour. HOWEVER, since I redid my tank I have made a few dead spots with my LR so I'm going to get a smaller power head for this one problem spot I have that has almost no current from rock blocking the flow. Flow is very important in the interest of keeping your tank clean.
~Scotch :.