My favourite way of choosing a surname for a couple/group of people who want to share a surname, is to merge their surnames, or even to invent a new one from scratch.
So, for example, Adam Smith and Diego Morales would become Adam and Diego Moralith, or Smorales, or any other combination of the two surnames. You may want to avoid choosing one which will send everyone off in giggles, unless that's your thing. If you plan to have children, try and choose one which won't make them the laughing stock of everyone they meet.
You can even invent a surname from scratch. Maybe there's a word which means something to both/all of you? Do whatever you like!
Merging or inventing new surnames tends to work better than a double-barrelled surname for all partners. There are various reasons for this. Firstly, women still find that their choice of a double-barrelled surname is not always respected as some people will think it's ok to call her by her first surname only. Double-barrelled surnames also have the disadvantage of being long and unwieldy. And, finally, with double-barrelled surnames, the question remains: which one comes first? And, if the order isn't the same for all partners, which surname will it be for their children, if they do decide to have children?