To tell you the truth, the settings I have my (2) set at, power-wise, is only slightly more than one at full power.
The pros of two: 1) Variable chaotic flow patterns 2) at full speed the MP10 is loud, two at 60% are silent. 3) one MP10 at full power kicked up tons of sand and caused the "whirpool" effect and sucked in water from the surface like crazy and blew bubbles - if I lowered it to eliminate the whirlpool, then my sandbed was toast. 4) I have to believe that the 2nd PH gets into the other side of the rocks, which may not have had sufficient flow with one allowing for a buildup of nasties on one side of the rockscape.
That's all I can think of. For the above listed, I'd say for me the importance of a 2nd PH was a 7.0 - 7.5 out of 10.0.