I have a unique situation I need some guidance on. My FI was born from Mexico where they have 4 names. First, middle, fathers last name, then mothers maiden name. So he socially goes by his middle name and mothers maiden name, however for work and financial and legal items he goes by his first name and fathers name. All of our friends know him by his social name. My question is, can I take his social last name? It's on his social security card and drivers license, so I would think so. Down the road I want to make sure our kids and I do have the last name. Insight?