I think that would cause a ton of lag. ^_^; It'd be like having one extra pair of fighters moving around, and everyone in the server will need to be able to see that, according to your idea. If you have a lot of people in the server, and let's say everyone is doing advanced melee.. there'd be tons of those 'shadow players' - the ones that appear to be moving to everyone else - and that'd lag the server like there's no tomorrow.
The idea isn't too bad, but I don't think it's worth all the trouble of coding it in. ^^;