Hi congrats! I'm expecting twins too and though I've had experience in BFing my elder girl, I'm quite nervous as I have no idea what to expect when my twins arrive!
In the beginning, yes you do feed on demand to stimulate supply. From what I've read, since BFing twins means double the supply needed, you may have to pump after each feed. Or you can pump one side, while latching one baby on the other. It also depends on whether you want to feed both twins at the same time, or really wait till they ask for it. But I think it'll be VERY tiring if one twin feeds at 2pm, then the next feeds at 3pm. My advice is to try to adjust and see how it goes, cause that's what I'm intending to do!
Breastmilk will have to be consumed within 48 hours in the fridge, but I always play safe and have it consumed within 24 hours.
For frozen breastmilk I think it can last up to a week.