Nah, I don't think so. While it makes sense for the player to check it every second so (s)he doesn't exploit it (giving that it won't be a clickable option like Tocan suggested above but alas, many item variants), the NPCs are only switching between their weapons at specific occassions (like the range towards enemies). Having them for two seconds the big shield in the same hand as the bow wouldn't break the immersion a lot, given that you normally would also first pick the bow from your back before putting the shield there.