Iain, you can currently add multiple client contacts for a client. The typical usage with the client contacts is to have multiple jobs for the client with a different client contact for each.
I'm not sure if this satisfies your feature request. It looks as if your issues might be more around who the invoice is emailed to. We will be improving the flexibility of this in a future release. Perhaps you could take another look at adding multiple client contacts and put another comment in here if this doesn't resolve your issues, with some more information on what you need.