You can combine baggage if you and your traveling companion have a single Booking or if you are traveling as a family. At check-in, an employee will weigh both bags, and if they are not heavier together and one bag fits the norm of 23 or 32 kg (according to the standard indicated on your ticket), you will not have to pay extra. But this rule applies only if both passengers are flying at a fare with baggage. With the "Basic" fare, you cannot combine your luggage with the baggage of your travelling companion.
If one of the shared bags is heavier than 23 or 32 kg, you will need to pay extra for extra baggage. Read more about it here.