You will pay per bag, up to a certain weight limit.
You cannot then split that across two bags unless you are given the option to do so on booking, in which case you will still pay per bag and the total weight is not likely to be much higher.
If you mean pooling your allowance, ie 2 people, 1 bag each, weight limit per bag 20kg, then you cannot take 1 bag weighing 30kg and 1 bag weighing 10kg. They may let you get away with a 19kg 21kg split .
You cannot put it all in one bag. There is a maximum weight per bag.
The advice given above is possibly not correct - I advise you check with your airline are the rules are different depending on who you fly with and what ticket you have bought.
For example, short-haul in economy with BA allows you one checked bag of 23kg each, if you were flying with your DH or whatever and you turned up with one bag of 30kg between you, you'd get charged excess at whatever the airline's rate is even though one of you didn't use your allowance.
If you are flying with an under 2 they don't get a baggage entitlement.
Also, remember that no single item can be heavier than 32kg (any airline) due to health and safety restrictions on what baggage handlers are allowed to lift.
DD was just a few months old when we flew with BA and she had a separate suitcase with 23 kg. (it was a 2.45 min trip so not longhaul 2 years ago) Also airmalta allowed 10 kg extra in my suitcase for her. You have to check with the airline. These were all the cheapest tickets, nothing fancy.
The maximum weight is just that. If it is heavier you pay excess.
Two people can share a bag but the maximum weight remains the same and cannot be combined.
The charter and budget airlines are very hot on this. Ryanair will let you book either a 15kg bag or a 20kg ( more £) per person. Jet 2 is 23kg. BA as above. Monarch is 20kg. They're all different. Check the airline for hold baggage charges.
First part of my post is not very relevant to you op, just answered the poster above about BA (infants get their 23 kg without a seat) But when I called them about pooling the guy couldn't tell me. so it was safer to take one for her instead just turning up with a heavier one. Budget ones don't do pooling.
Usually baggage allowance is based on how many bags you have with you.
So say each person is allowed 20 kg, if you bring the maximum allowed, you can have 80 kg, split across 4 bags. If you only bring 2 bags, you can have 40 kg split across the 2 bags, if you bring 3 bags you can have 60.
Each bag does not have to be exactly 20 - you would probably be allowed, say, 17 in one and 23 in another. However there will also be a weight that a single bag is not allowed to exceed (think of the poor baggage handlers!). This also depends on the airline - it might be over 30kg, it might be 23kg.
Apologies for giving duff information! I've always travelled with the kids (older kids so with baggage allowance) and split stuff into two cases but they were never especially heavy so I suppose they were within the weight allowance for each bag.
We just flew with Thomas Cook and my understanding was was had 15kg allowance each, so 45 altogether for us, to be split however we wanted as long as no single bag was overa certain weight for health and safety - I think it was 40 kg. However with the hand luggage they were specific that it was one bag of max 5kg each. Definitely check with airline!
Flying with easyjet recently, we had an 80kg allowance spread across 4 bags. Some of our bags were heavier than 20kg, some lighter. The heavier ones (which I think weren't allowed to exceed 32kg but not sure) were labelled as heavy bags, but it was total weight that mattered. Read the small print, the airline will tell you.