I'd rather just enjoy their already higher attach speed and fire them simultaneously. It should be a passive, or choice, to trade the 15% bump from dual wielding to actually firing both hand xbows at the same time.
It makes zero sense to fire dual wielded hand xbows the way the demon hunter does. In most cases it would balance out on dps if fired simultaneously, unless you have two of the best legendary lvl60 hand xbows. Let's face it though, that would be so near impossible to accomplish it would just add to our never ending chase for loot anyways. Plus, a good quiver+2 hand bow would then have the same attack speed.
The craziest part about all of it is, then our stupid statistics would make sense again and we wouldn't be basing things off our undocumented ingame perception of "other things happening faster".