You might do much better with a better pump - I've used a Medela Pump in Style and find it works better than the smaller pumps I've tried in the past. I can only get an ounce at most with the others but with the Medela, I can fill a 6oz. bag.
Don't worry that if you pump too much there won't be anything for your baby to nurse. Your breasts aren't just storage areas for milk, they are "factories" - the milk is being produced constantly, even if they feel empty, that doesn't matter. Your body will recognize how much output is required and will increase your supply over a few days. Pump as much as you need, nurse as much as you need - your body will meet the demand. Make sure that you pump each time you feed the baby a bottle, or you are sending the signal "don't need milk right now, please make less" to your breasts.
Remember how your baby increases your supply - by nursing more frequently during growth spurts. Pumping in between nursings will have the same effect and you will have enough milk for both.
Check out www.kellymom.com for a lot of great info about pumping and milk supply, it's a great site! :)