My two children's names both start with the same letter. This wasn't intentional but I know of others who follow patterns like that.
I only had one rule which others thought was weird. I would not choose a longer name for ds2 than ds1s name. To me, if I see the names Ben and Christopher, for example, its weird the second child having a longer name than the first. I therefore would only consider names 5 letters or less for my second child. My dh thought this was weird!