Opening one of the chests makes the corresponding chest in the reflection side into a mimic chest. But if there is only one key on the level, then what is the point of having a Crystal Mimic you can never reach?
Also, symmetrical levels are really boring in other games I've played. Once you clear one side, clearing the other side just seems like work. Plus, this would be a pretty difficult code change considering how SPD generates levels and I currently don't see the benefit of this type of rare level to make the change worth it.