I imagine a shared sub with no change in price from the normal FFXIV sub probably wouldn't happen, but some form of discounted shared sub would be neat. Something like the FFXIV (or FFXI) sub price +$5/month and you get access to both games.

I've had a mild interest in trying out FFXI for awhile, but unlike XIV, the free trial doesn't get you too far past beginner stuff unless you hard commit to playing a lot in that time.