Attached Link: http://www.buckrogers.demon.co.uk/3d/thumb/rs.jpg

I don't know about old-type knight-in-armor and ancient warrior shields, but I have used modern riotshields and they are parented to the left forearm, not to the left hand. See this link (the thumbnail of my riotshield model). But the goal of the left arm's IK-chain is the left hand, not the left forearm. That might complicate matters. If you set the sword arm IK-on and IK-parent the sword hand to the shield arm hand, the sword hand will follow the shield arm hand, if it can reach, but the shield arm hand won't follow the sword arm hand. Or I think so. But you can't change IK-settings or parentage during a Poser movie run.