Ive read that you can only mix them once it's all the same temperature, so you would have to pump separately, then consolidate once it's all refrigerated.
At the BF/Pumping class I went to tonight, she talked about combining once they have both cooled to same temp in fridge, like PP said. The woman tonight said you could pump today and put in fridge...then pump a little more tomorrow and put in fridge, then combine (so it doesn't even have to be milk pumped in same day).
I've combined mine once it has cooled to the same temp. I try to combine same day pumps or pumps that were only a day apart. That way I can easily keep track of how long the milk has been in the fridge.
Re: Combining milk of multiple pump sessions