Yay! I introduced my girls last night. It was a good few hours of closely watching them, and stressing just a wee bit.
First I swapped their cages for a while so that they could get used to the new smell.
Then I put them in the bath...just a little bit of water (about an inch) with some play toys and a little dab of vanilla on their backs. They were so interested in the water that they weren't too worried about each other. Someone else on here suggested that! Thanks!
I put them both into a completely new wire cage with new toys (goodbye aquarium!). They freaked out a little bit at first. I kept breaking up their fights and then I realized that they weren't hurting each other really...it was more like they were trying to establish who was in charge. So with my gloved hand and my sweaty brow I nervously watched them "fight". It was probably quite a sight! But I am pleased to say that they seem to be doing fine today and seem to be having a great time together. No more lonely rats for us! Yay!