If I understand your question correctly, Mami usually takes the lower harmony and Tomomi has the higher vocal range(other than Rina) so she would be the high harmony part.
It depends on the song. I was surprised during the latest live DVD (Perfect World) to see that Mami actually sings some of the extremely high harmonies - I can't remember which now, though, but I saw it in a couple of songs. Always possible she could be using an octave changer or something, though. But she could just be singing in falsetto. It doesn't really sound like her when she does it, but on the disc you can see her mouth movements match the sound of some of the high harmonies.