Thanks Leagen, I'd noticed that too, although I didn't know that was what the Spanish did.
Being half Italian I get lots of post addressed to me in the same way, with my maiden name and married name, so it's pretty similar to the Spanish way although I don't know about using them as middle names. I just found it really odd that two of my old English relatives seemed to begin using both names later in life, having started off with only one. I didn't expect to find people on both my mother's and father's sides using a similar system! Having said that, it's been enormously helpful when I've come to find their parents. I'd just love to know what prompted them to do it.