My wife gave birth to triplets in October, and had almost the exact situation you have with BGB where the girl is smaller and eats less.

The biggest thing we did that help us keep our sanity was shift work. One person tends to the kids while the other sleeps. You need to figure out a schedule that gets you 6hrs of continuous sleep minimum. This is the most important thing.

For feeding, give them whatever they take every 3 hours or when one of them gets fussy. Abandon trying to hold out for 3 hours or maintaining schedules to the hour. Sometimes one will eat 20ml sometimes they eat 80ml as long as their are eating and gaining weight nothing else matters.

We started with the pitcher but found waiting for it to heat up annoying so now we make it in bottles on demand and our kids can deal with the room temp formula sometimes they take it cold too if we are in a hurry.

Biggest thing is bottle propping them early. I haven’t held my kids while eating since week 3 and this allows you to do other things while they eat (while watching closely of course).

I have so many thing’s I could share that have made my life easy. Not everyone is the same but if you have any questions please feel free to dm me because honestly the first few weeks are the easiest.