If you keep the first one in the fridge, put second in fridge and then when it is cool add to the first.

Are you freezing milk? I would only freeze in smaller amounts (30-50ml or less) if you using as top ups, so you don't have to waste any. Can you get some ebm bags or freezer trays for storage?

https://www.breastfeeding.asn.au/bf-...ing-breastmilk