While researching my Derham ancestors, I found a number of people suddenly popped up with the Gaymer-Derham surname. I couldn't find a marriage for a Gaymer / Derham couple which I guessed should be pre 1910, because that was the birth year of the oldest person with the double barrelled surname. Nor could I find a birth for him in 1910. But I had found death record, records of two marriages and record of death of what I thought was his second wife. It turned out he actual had three marriages, the first time he married in the surname Derham, so the lady I thought was his second wife was actually his third and he had double barrelled his name between the first and second marriages. I did read on-line somewhere a long time ago, that when he joined up for war service he had a very good friend with the surname Gaymer, and they agreed if either were killed the other would take his surname. But I can't find that article now.