What the passport office says#
GOV.UK's guidance on a passport after marriage or civil partnership says to send your certificate when you apply for a passport in your new name, and then adds, in the same sentence, "this includes double-barrelled names". A couple who marry as Smith and Jones and become Smith-Jones can each get a passport in that name with the certificate alone. The certificate does not show the double-barrelled form; the passport office accepts that the two names on it, joined, are what you now use.
What everyone else says#
Most organisations take their lead from the passport office and accept the certificate for a double-barrelled name. A few want to see the exact new name written in a document before they will record it, because a certificate shows two surnames rather than one joined surname, and a member of staff may not be sure which form to enter. If that happens, a deed poll in the double-barrelled name is the answer; it is accepted everywhere and removes the judgement call.
Hyphen, space or neither#
You can double-barrel with a hyphen (Smith-Jones) or with a space (Smith Jones), and HM Passport Office accepts the marriage or civil partnership certificate for either. Running the two names together as one word (Smithjones) is different: its guidance says that makes a new name rather than a double-barrelled one, and asks for a change of name deed as well as proof you use the name. GOV.UK's deed poll rules allow a hyphen and say a new name must be pronounceable and free of symbols, numbers or punctuation apart from a hyphen or an established form like O'Hara. Whichever you choose, use it the same way everywhere, because "Smith-Jones" and "Smith Jones" can be treated as different names by a computer system.
Which of you changes#
Each partner who takes the double-barrelled name provides their own evidence. If only one of you double-barrels, the other's records stay as they are. If you both do, you both send the certificate, or you both make a deed poll. Changing your name after marriage has the rest.
For a child's surname, see giving a child a double-barrelled surname. To drop one half later, see dropping half of a double-barrelled surname.
