That sounds really interesting and would probably improve the behaviour of a lot of creatures but I'm going to be honest: Implementing this would be quite a lot of work so it's probably not going to happen.
To be a little more specific, parts of the code that would be affected by this would be:
- The creature building logic. I'd need to have a way to figure out which muscles are a symmetric pair. (There are a bunch of edge cases and problems that come with that)
- The creature saving logic. I'd need to add this additional information about symmetry into the save files, which would then cause me to have a new save file version which needs to be distinguished from the old ones.
- The runtime logic of how the brain connects to the muscles.
- If I implement synchronized symmetric muscles, why not just allow the user to pick arbitrary groups of muscles that will then receive the same brain outputs, instead of restricting it to symmetric designs.
This last point is actually quite interesting and could lead to some really cool results. I'm now a little bit more tempted to do it. It's still going to be a large amount of work and is most likely not going to happen in the next year or so because I just don't have any time, but maybe with the next big update (if there will ever be one).