That's an interesting question. My experience leads me to go with that old engineering standby: It depends.
For sites that can easily translate between platforms using CSS, etc, and its not a gnarly mess, them that seems to be a good approach. Once the pain of maintaining the data in one place exceeds the value then splitting seems called for. Hopefully the improvements in maintainability and understandability outweighs the effort that may be required to change things in two places.
Is it possible to keep the common things in a single file and the platform specific items in another file (or one for each platform)? I haven't done anything big enough to know whether this would work or just cause a different mess.