behavior:
if you have an ranged item with critical chance and / or crit damage in the offhand and an melee item with critical chance and / or crit damage bonuses in the mainhand, the bonuses to critical hit chance and critical hit damage will stack.

expected behavior:
consistent with behavior when you have an ranged weapon in the mainhand and an ranged weapon in the offhand (offhand weapon grants no bonuses)

This is just how minecraft's attribute system works. It's not feasible to make bows only provide their bonus in the offhand, since many players will use bows in their mainhand, and dynamically adjusting the valid equipment slots depending on if you are holding an item in your mainhand is unreasonable.