1st - chill your 50ml in the fridge (not the freezer one). 2nd - chill also your 2nd pump in the fridge. wait for the 2nd pump to be cold, same temperature as the 1st one.
finally combine both and then bring up to the freezer compartment
but.... remember to do this within 24 hours, better if within 12 hours for freshness.
i pump twice in the office and chill in the fridge. so the moment i come home, i will freeze both by combining them and put in my milk bags.
stored EBM must be consumed by 3 months max .
hope above helps. cheers!