I'm going to assume that you're talking about archers that have shields on their backs when they shoot, then take them off in melee. That's pretty simple. You'll want to use "shieldactive0" for the shield paired with the unit's primary weapon, and "shieldpassive0" for the shield paired with their secondary weapon. Most of the time, you'll want to use "shieldactive0" on the back and "shieldpassive0" in the hand. For instance, your comment on the back shield might look like this:
...which will be paired up with this:
Just be sure that both "buckler_01" parts share the same model and texture, and assign the shield on the back to the skeleton's torso. It doesn't just work with missile units too; in Broken Crescent, some heavy cavalry units have the shield on their back when they have their lances out, and grab their shields in extended melee. Also in Broken Crescent, the Varangians carry their shields on their hand by default, but put it on their backs when they charge into melee so they can use their axes with both hands.