I assume you are hoping to drive both shafts from a single input. In that case why not look at the Schmidt coupling design. This allows infinite float within the operating range with no speed variation and no loads on related bearings. It would probably be desirable to have some limiting rigid...
You could look in your public library. In UK all public libraries keep these standards or allow access. If this is a no-no then give me a fax number and I can send hard copy.
British Standard 4235 or DIN standard 6885/1 will provide standards for metric keys and keyways. BS 46 dimensions commonly used Imperial keys and kkeyways.
