I love the idea of this parasitic twin costume. I've been to quite a few big Halloween parties, and never seen anyone in a costume like this. Probably you should come up with a witty or quirky name for your "twin," but you probably shouldn't try to use it as a ventriloquist's dummy.
Your pre-teen or tween can also go as a parasitic twin. I bet they'd like to jack up the grossness factor by adding drippy green or red jello to their baby doll part of the costume, too.
Also available on Amazon.