My sister in law has shared custody, as does my sister.
In both kids, I see a bit of distress and turmoil that I don't see with kids who live in one house.
Maybe it's the constant change, not having one bed all the time, having mirrored families, and constant car rides.
They develop faster verbally (from having to explain what they want to 2 diff groups), but emotionally I do see strains. (I have my BS in elem ed and studied early childhood development a lot).
My advice to you is to try to make it work. Talk it out, get counseling, try to fix things. Even if it means you only get another 3 or 4 or 5 years as a standard family unit before splitsville. however, that child being loved and having a calm environment is the most important thing, so if there's constant and loud fighting, then maybe it's time to break. Good luck.