Just one little thing to clarify. I think it is best to do an experiment. Get a box of formula now. follow instruction on the box, 1 scoop for how much water? mix it and see what it comes up to on the bottle. Write it down. (it might make more sense to do at least 3 scoops, as you don't want to miss out on the round up/down.)
Then mix a jug the way you would want to do it. I think the cooling off would take considerable time, unless you use ice. (Get a feel of how long it would take, so you will not get a shock). Then after proper chilling test how much cold (from the fridge) and how much hot water give you a good enough room temp bottle. 60ml of 5C water will not need 60ml of 100C water to come up to 23C. Just the math doesn't sound right
AND when you run out of premade one, what are you going to do?? What is the contingency? Pour hot water, add formula, stir, then add cooled water!
That, to me, is a must know-how!
Oh and Laura, I wanted to find your other thread but I will just add here. As I had awful back pain that I couldn't even hold DS as a newborn, I sometimes fed him on a nursing cushion, the kind of microbeads, not the hard filled ones. It can nicely hold a baby in place but not too high, esp. as a newborn. You will have a hand free for twin b. You probably can prop them both next to each other and hold both bottles. Soon enough, they will be able to hold the bottles themselves.