steve, it's at my parents' house. They took the old tank, filter, and heater in case something ever happens and we need to move the turtles when moving the bigger tank is not possible (something like when we thought we may need to evacuate for Sandy).
CactusJohn5, it's mainly when food comes into play, so I separate them during feeding. I don't know why Taro is so aggressive, I've made sure they have plenty of food. I guess I should have started separated feeding sooner.

With the divider (the divider is higher than water level) they shouldn't be able to see each other eat anymore. It's not that Taro outright attacks Yawara - it's that she is like a seek-and-consume eater. Is he coming back up for his next pellet? CHOMP - no he isn't - she just swam the length of the tank to take it from him and is now trying to see where I will put the next one.
I will offer my friend more money and see if she will reconsider watching both. It seems like she's not afraid of Taro, but afraid she did something wrong last time, which is doubtful. I think I just should have gotten the divider sooner. Thank you all for the insight!