She's too small to take adult mice, but you could definitely feed her two fuzzies in a row. Just feed one, wait until it is swallowed, wait a few minutes, and feed the other one. It's best to feed an "entire meal" within the same day, not spread out a few days apart.
Though if she has already been eating consistently with you, I would order rodents in bulk online. It's much cheaper and better quality than petco, and you can get the sizes you want. She should be eating hoppers.
Big Cheese has the best prices: http://bigcheeserodents.com/