Complex sentence: Rem explains to Misa that one cannot see the life span of someone else with a Death Note; this is why Death Gods cannot see each other's life spans and why Misa cannot see her own life span.
Split into simpler sentences: Rem explains to Misa that one cannot see the life span of someone else with a Death Note. She elaborates, telling Misa of how Death Gods cannot see each others life spans and why Misa cannot see her own life span.