You can go to this link to check out the light distribution for DE pendants. It would suggest that mounting DE pendants in parallel is best. The charts show the output of light under the reflectors with the length of the lamp being on the Y axis. "A right hand coordinate system was used to designate the points on the grid, with the longitudinal axis of the lamp defining the Y-axis" - Part I.
http://www.advancedaquarist.com/issu...03/feature.htm
So you can see that placing the reflectors/lamp lengthwise in parallel would be my choice.