I'm personally not a fan of two middle names but if you do, I think you should give all your children two or one. Unless you do girls get two middle names and boys get one, etc.
My only exception is adoption. I think keeping at least one of the child's birth names is important.
Well, right now we're just brainstorming names, seeing what works and what doesn't, which names we'll never get tired of and which are just names we have a crush on so to speak, but some have two middles, some one. We're only getting started on the boys' list, but the girls are pretty much done. Just a little more tweaking (Pandora needs some work I think). Some have two middles, some have one. We like the name to tell a story, paint a picture, sing a song (whatever), and when it's done, it's done. Sometimes the story needs more to finish it off, other times what it has is perfect. If that means some children have two middles and some one, so be it. Hopefully there's no insecurity or jealousy with that.
I think I'll be painting a visual interpretation of their name once the child is here and they're named, so I hope that helps them understand what their name is. Tabitha Eilonwy Snow, for example, is a castle (that's a very brief interpretation, but that's the gist of it). If I took the Snow or the Eilonwy out, the painting wouldn't be complete. Lorelei Ondine is the sea. If I add anything to it, the painting changes. It might not be the sea anymore, it could be a pond or a fish, or who knows what, but she wouldn't be the sea I've envisioned.
I'm not sure I've explained it right. :) In short, I hope my children are as weird as I am and they get it.
Vivienne Lux is gorgeous, by the way.
I grew up with my brother having two middles and me having one. It never bothered me. When we had our first we decided we would use the middle spot to honor family, and I really wanted to honor my late father, but also wanted to include my husbands name. Our initial plan was to use one of the names for the first child, and if we ever had another boy, then use the other. I decided that if we never had another boy I would regret not using both, so we did. Th same thing happened with our second baby, a girl. Wanting to honor both sides of the family we again went with two middles. If we had only used one middle the second time around, I would have felt no pressure one way or the other or the third child, but now that we have two, each with two middles, we will definitely be giving the third child two middles as well...trouble is were running out of family members to honor!