Doo eet!Actually this change wouldn't make such a combination very powerful at all. Strong versus arrows, weak versus infantry, prey to cavalry.
Maybe just decrease the Field of Blocking while on back. If not a Tower Shield will make you almost unshotable which will lead to unbalance.
Bring back the old native system, where are shield on your back can block arrows, not just lower the dmg...
I believe that's because the armor soak, shield armor and archery damage differences between Native and cRPG cause arrows to systematically glance on a back shield in Native but almost never do so in cRPG.I can confirm this as I remember quite a few times in native (and mods) that I got the "hit shield on back" yet still took some (low) damage.