With the recent inclusion of the Spirit King Ring (or however its said, the diamond dust one) and that it does stack twice, I believe that perhaps we have some leeway to argue that double Dex part should give full axe boomerang reduction -- especially because it does not claim it to be part of the combo.
edit: well, I tried asking. apparently the world is not ready for axe boomerang spam.
Are we sure that they don't already stack? Axe Boomerang has a 1 second cast delay in addition to a cooldown, so even with 2 accessories, strings would still be required to spam it.