Ah. I think I misread the question.
although if I recall correctly, the 'two suns' was only established in Metru Nui. And when the Toa and Matoran traveled from Mata Nui to Metru Nui, the Great Spirit's eyes would still be closed - but there was effective daylight in Web of Shadows.
Which raises the question - where do the other places in the world, such as Karda Nui, Daxia, or Zakaz get their light?