I think that is great that baby #1 is wanting breast milk. He is nursing then? If so, he's still so young and htat is perfect for him. Breast milk is just fine and they don't need dairy in place of human milk.
As for you not saving the milk for the baby in the NICU, we had a similar situation with our daughter when she was born. She had open heart surgery at a few days old and I pumped and pumped. I had SO much milk! Even when she started having more milk, I still had way too much for her and had to eventually toss out a ton of breastmilk. Just wanted to mention this because I think you will be just fine giving baby #1 some of your breastmilk. It's very good for him and he's still a baby too!
I would just make sure to eat a lot and drink a lot to make sure you aren't sucked dry with your milk supply;-)
I hope all goes well with your sweet little one who is in the NICU and that she gets to go home soon.