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1. can I mix milk I pumped from both breasts into one bottle? 2. also, can I mix the milk from two different pumping sessions?

1 - yes you definitely can 2 - yes you can - but, I have read certain sources that say it should be from the same 24 hour period, so for example you shouldn't combine milk you pump on Monday with milk you pump on Wednesday. Also, if you are putting in the fridge, go by the earliest date when determining how long you can keep it (generally I've read it stays good for 5 to 7 days in the fridge, so go by the day of the first pumping session)

yes, as long as it's within 24 hours

Yes, you can combine milk. Just make sure to cool the newly expressed milk before you add it to the previously expressed milk. Also, it is helpful to gently mix in the separated fat in the previous milk when you add the cooled new milk. When I first started pumping, I only got an ounce or so out and it took about 2 days to fill a storage bag. If the bag isn't full after 36 hours, mark it with the amount that's there, freeze/ use what you have and start a new bag. Don't leave it in the fridge longer than 8 days.