As I understand it, when entering the EU from outside, you can't combine your duty free limits on a single item. So, I believe, two people can't bring in a single 2l bottle of spirits for free, as the allowance is 1l per person, and that can't be combined.
However, if there are two of you, you're both allowed to bring your own 1l bottle of spirits (or other things within limits)
This is slightly different though, and applies to bags. Suppose you're outside the EU with a friend, and spot an interesting looking local drink. You each buy a bottle. However, you only have the one piece of checked luggage between you. You can't take the big bottles through airport security, so you pack both bottles carefully in your one piece of luggage, and check it in.
What happens on arrival though. Are you able to go through customs together, with the one bag, and if questioned say the bag is a joint one and point out who's bits are who's? Or does one person need to remove their bottle from the bag between baggage claim and customs, so that no one bag is over the limit?