You can mix milk from different pumps. But depending on how much is in each bag, it might be better to offer it separately and see if it's needed... if she doesn't drink a lot of EBM while you are at work you don't want it going to waste, it could be offered next feed then.

Depending on how much milk you get from each pump I don't know if I would be expressing after every feed, maybe try 3 or 4 times a day & see how you go?

ETA - just re-read ... if you're going back to work in a couple of weeks, maybe express a bit more atm to get your stash of EBM up & then reduce if you can, once you're in front & you know how much she's going to need.