If my flight ticket type allows me to take with me a maximum of 23 kg luggage, can I take two luggage with me whose total weight is less than 23 kg on the back of the plane?
For Lufthansa, the situation is complicated enough that they have a Free baggage calculator.
For an Economy Class passenger with no elite status flying from New York to Berlin the allowance is 1x Carry-on, 1x Personal item, 1x checked baggage, with limitations on size and/or weight stated for each. You should enter your actual flight information but there will be a limit on the number of items as well as on weight. For the case I tested, I would expect a surcharge for more than one checked item.
You should check each airline.
This varies a lot.
It used to be that there were two concepts:
piece concept: you are allowed a number of pieces of luggage, and there’s a max weight per piece. This was most common on flights to/from the US
weight concept: you are allowed a total weight, and there isn’t any limit to the number of pieces.
Nowadays it’s a lot murkier, and it really depends on the airline. Many will give both a total weight limit and a limit on the number of pieces. Others may still use either of the two concepts above. And of course this may vary based on the flight (domestic / international, and based on origin or destination), class, fare...
You need to read the exact rules as they are specified by the airline. If you tell us the exact airline, flight and fare we may be able to decode this for you.
You can find this information on the airline websites. There is no standard across the industry, it depends on the airline. And the ticket/fare type, and also the route, even a single airline this can vary by the route- traditionally airlines have had different rules on luggage for flights to/from North America than flights within Europe, for example.
Many airlines, it is by default both a piece AND weight limit, you cannot combine multiple pieces. This started with low-fare airlines but it is has spread now as a standard to full service airlines and all three of the airlines you mention the default limit on the cheapest fares that include luggage is only 1 piece of luggage, and you cannot spread the weight over multiple pieces.
You often have the explicit option when booking if you want to book one bag, two bags, etc. So what you end up with will be exactly what you pick. But if you really want two bags, you need to make sure you book two bags when buying the ticket.
Alternately, I have done the trick suggested by @CSM of tying together two pieces of luggage to make them one piece many times.
For the specific airlines you mention:
The lowest fare for Eurowings is the BASIC fare, which includes no checked baggage.
As a saver fare, our BASIC fare includes the flight and one free piece of hand luggage.
These are the luggage rules for the other fares on Eurowings. As you can see it depends on the fare but the SMART fare it is ONE piece up to 23kg only. Fares above that give you two items.
What charges apply for checking in luggage at Eurowings?
You will be charged for each item of luggage that you check in. If you check in more than one bag weighing 23 kg, an additional fixed charge for excess luggage applies. ...
Luggage can be registered at the time of booking the flight or at a later date.
SMART fare Passengers who have booked the SMART fare can check in one item of luggage weighing 23 kg (no special or excess luggage) at no extra charge.
BEST fare Passengers who have booked the BEST fare can check in two items of luggage weighing 23 kg (no special or excess luggage) at no extra charge.
BIZclass fare If you have booked the BIZclass fare, you can place two pieces of luggage of 32 kg (no extra or excess baggage) at no extra cost.
For BA, their cheapest fare has zero bags:
If you are travelling on a hand baggage only (Basic) fare, checking in baggage costs extra.
Beyond that, the standard is weight quoted applies to each bag, so it depends how many bags you have booked. The lower fares include only one:
The weight limit applies to each bag. It's not possible to split the total weight across multiple bags.
For Lufthansa, within Europe, the lowest option, "Economy Light" has no checked baggage at all. The next option, Economy Classic & Flex, allows you "1 item of baggage up to 23kg". Business Class allows you "2 items of baggage, each up to 32 kg".
The rules from intercontinental are less restrictive, but the most basic "Economy Class" is still only "1 item of baggage up to 23kg". You need Premium Economy to get "2 items of baggage, each up to 23kg".
Nearly in all cases you are given a piece allowance first and then it gets a weight limit. So, if your tickets shows 1 PC or 1 piece, then you have only 1 bag allowance for checked luggage and you cannot split it into two. This is generally the case for economy tickets with the airlines you mention and a good percentage of others.
That being said, there is nothing stopping you from putting one bag into the other or both bags into a larger bag or wrap and it will look like one item. You will be given a single luggage tag to track. The only restriction is that the item must still fall within checked-luggage limits in size and weight.
I frequently fly into or out of Japan, for which a number of airlines allow a greater luggage limit than for other destinations. Essentially, for tickets to or from Japan you will usually either be allowed two pieces at 23 kg each or one piece at 30 kg. If the former, it will always be mentioned on the ticket as 2 pieces (PCS).
Applied to your situation, if the fare doesn’t explicitly state more than one piece you are likely only permitted one single piece adhering to the size limit of a single piece and weighing no more than 23 kg. If you are allowed multiple pieces at 23 kg each, that will be stated as multiple pieces.
Some airlines also use the weight concept for some or all of their destinations. Under this concept, the number of pieces is irrelevant as long as the total weight is below 23 kg. I remember clearly that I had a (Finnair) flight whose luggage rules adhered to the weight concept back in 2006 but every flight I took since at least 2017, probably 2010 or 2013 followed the piece concept.
If you want clarity, you have two options:
check the airline’s (or airlines’) conditions of carriage or other appropriate documentation and see what they write there.
call the airline(s) and ask.
Note that whether the piece or weight concept apply may depend on the destination. I remember having looked up one airline’s conditions of carriage not too long ago and finding out that the luggage concept depended on the destination but I forgot which airline that was and which destinations were different.
I flew in early February on Transavia airlines from Israel to Schiphol (Amsterdam). I was flying for a conference and I had a "roll up" backdrop in addition to my suitcase. I picked the backdrop up on the way to the airport, planning on putting it in my suitcase, but it ended up being 2cm too big! The weight worked out; together it was 15kilo which was within my limit.
When I got to the airport for check-in I just showed them the suitcase and the roll-up, and they weighed them to make sure it was within the limits. Then they took the suitcase and told me to put the roll-up with the "oversized" luggage. They didn't charge me anything, nor did they imply that they were doing me any favor; my luggage was within the limit so it was ok.
I had a similar experience in Schiphol; I had no problem checking the roll-up as oversized luggage since combined with the suitcase it was within the limit.
And at the conference itself, a couple of other presenters told me that they had similar experiences in other airports for other conferences.
I am not saying that this was indicative of official policy; but this was my experience recently.