It seems very likely that most ancient shield users with training / drill
used both their weapon and their shield offensively and defensively in
coordination. I'm uncertain that that might translate to in TFT with a
minimum of rule changes and in keeping the spirit of the rules. It would
seem to me that if you get two strikes, the -4DX for two weapons is in the
spirit of Melee. As any offensive motion with the shield would still leave
you behind it, I'm uncertain you should lose the defensive bonus (most
likely you are "punching forward" with the shield, leaving you behind it,
right?). In other words, the shield isn't an either/or attack/defense
weapon. I think the -4DX should be enough for what is, after all, a pretty
weak strike, 1d6.