As I understand it, the limits for duty free for both the UK and France for people arriving from outside of the EU is a maximum of 1 litre of alcoholic beverages over 22%. As such, on a single direct flight into the EU, one person wouldn't be allowed to enter with 2 litres of spirits without paying duty. However...
Consider a trip which was USA to Paris with a few hours stopover in London, then the next day a flight from Paris back to London. Assume two pieces of checked luggage.
On your flight from Europe to the USA, you buy one litre of duty free spirits. Before leaving the USA, you put that in bag #1. At the airport, you short-check bag #2 to London, and have bag #1 (with the duty free) checked all the way to Paris. After checkin, you then buy another litre of duty free spirits.
On arrival in London, you clear immigration, collect your short checked bag, clear customs with just the USA-purchased 1L of spirits, pop the duty free in that bag and leave it in left luggage. You then re-clear security and carry on to Paris. The next day, you fly from Paris back to London, with the other litre of spirits still in your other piece of checked luggage.
When you go to customs in London for the second time, with the other bottle, are you permitted to exit without paying any duty? Does the side trip to France with the second bottle reset your limit?