WWE simply doesn't have the depth of talent to fill two divisions
Actually they do. They might not have enough for two GREAT divisions, but they have enough for 2 divisions nonetheless.
RAW
Alicia Fox - heel enhancement talent
Bellas - face enhancement talent
Eve - potential title holder
Gail Kim - potential title holder
Jillian - heel enhancement talent
Kelly - potential title holder
Maryse - potential title holder
Melina - current champion
4 "enhancement talents" and 5 title chasers.
SMACKDOWN
Beth Phoenix - potential title holder
Layla - heel enhancement talent
Maria - face enhancement talent
Michelle - current champion
Mickie - potential title holder
Natalya - potential title holder
2 "enhancement talents" and 4 title chasers.
Then don't forget ECW has Rosa and Katie, both of which can be brought over to Smackdown as an enhancement talent and title chaser, respectively.
So... that proves every "there's not enough divas to work two full divisions" argument wrong. It can work out quite well because many of these women are better than what they are given credit for, it's just up to WWE to create the right storylines and make the fans care at the same time.