Change send mail shortcut to tab + enter in Apple Mail
Because tab isn't a modifier key (like command, shift, option and control), you can't use it in combination with other non-modifier keys to make a shortcut. It's simply not possible the way the OS X input systems work AFAIK.

You might be able to jury-rig something together using KeyboardMaestro or similar apps that listens for tab-return and then executes the send mail command, but I don't know how well it would work, or how closely it would match your expected behaviour.