I think the more sensible solution may simply "Fill" the slot with a duplicate of the main hand icon when using a two-handed weapon or item. You don't have to worry about reworking anything for PLD or Crafters/Gathers other classes don't have an annoying empty slot. However this is a super-small cosmetic problem, and any solution would probably be non-trivial to implement from dev resources standpoint. Not sure it's the kind of thing that really begs solving.