User blog comment:SolZen321/Ginga and Lugiel Theory/@comment-1733689-20150116005432/@comment-1733689-20150116012243

This is what jp.wikipedia says:

Time immemorial, has changed all of the Ultraman monster to spark Dolls in dark spark Wars, all of ringleader serving presence. "I stopped when all of life, to build the" eternal paradise "Naru peace" that is the purpose. But only "Stopping in happiness," "hatred is old does not feel faded" I have claimed the "stop" and by good intentions. Paired with Ultraman galaxy, and the original is the same also in the presence " ties of life going inherited eternal life = to posterity were split without believe the "light" of the galaxy that believe  in "," shadow " what is his true identity [Note 24]. Therefore or tone of voice is also similar to the galaxy, has a appearance,such as the black Ultraman to clad the armor, in  addition to the use of dark spark as a weapon, a powerful light bullets from breast red light emitter Dark-saving El beat, systemic I shoot the red destruction rays Dark-saving El shoot from.

Make of this what you will, but it sounds to me like they're more like yin and yang and were never "one being" but rather they were in sync with the Ginga Spark, then Lugiel felt the Dark Spark suited him better.