I currently have two children with names I love- Daniel Henry and Elsa Catherine. I am pregnant again and am considering these names: William Henry and Clara Catherine. The thing I like about the middle names is how well it flows with our last name, Hyde. We do call the kids by all three names quite often. My spouse thinks this idea is nuts, and I am just wishing that I named our son William instead Daniel and our daughter Catherine instead of Elsa. He may be right, I may have to mourn the perfect names William Henry Hyde and Clara Catherine Hyde.
Unless the Swistle Baby Naming Cohort thinks its not too odd to share middle names.
My vote is that it's not too odd! I think of the middle name as a perfect place to...PLAY, I guess. It's where you can do things you wouldn't do with a first name: for example, use names that are not really your style but you love them anyway, or use names that have deal-breakers such as bad nicknames or bad initials or a problem with your surname, or use names that are too way-out-there or too overly-common for your preferences, or use names that would have clashed with sibling names---or use duplicate names.
The one downside is that I think once you do it, you have to do it for all the siblings of that sex (and maybe also for the siblings of the other sex), which can make first-name selection difficult. You don't actually, literally HAVE to, but the pressure would be intense, and NOT doing it would take it from a fun "This is a cool bonding thing we do in our family!" (with the fun possibility that the children might grow up to do a version of it with their own children) to an odd "Hey, how come they get their own names and I don't?"