just curious, but how are you determining when she's “done“ at the breast? are you offering both? and then back to the first? why do you think you don't have enough milk? is it just because she “takes“ the top up?
this isn't a simple and quick answer, but it is a fixable situation!

remember, babes tummies are usually no larger than their fist, so if you are giving lots at the breast and then the extra ounces afterwards, likely she is really over full. that's a sensation she has to relearn.
keep chatting about it, once you get all the details and indicators straight, things will get simpler!