It would be much less awkward if Rangers and other Two Weapon Style combatants could have a button or other main interface control to switch from bow/crossbow to their preferred two weapon combo.
Ideally it would be scripted to move the missile weapon to a vacant general gear slot, but I wouldn't actually mind too much if it just dropped the missile weapon; there's a certain nod to realism there.
The off-hand weapon can't be in the off-hand box while employing the bow, etc, but I would not mind a requirement that the off-hand weapon must be the weapon in the last weapon box, from which it would automatically move to the off-hand box when the weapon switch was ordered. That requirement would actually automatically limit this capability to warrior types. (A little unfair to some rogue types, though; perhaps something else could be worked out.)
If this move seems too beneficial to Two Weapon Style, or too "unrealistic" to allow to be entirely smooth, penalize the weapon speed on the round of the switch, or allow only the switch in a round rather than an attack.
Let's admit it. All of us who play a Two Weapon wielder pause the action as a melee situation closes in, open the inventory page, move the missile weapon to a general gear slot, move the off-hand weapon to the off-hand slot, prioritize the main hand melee weapon, redirect the character's attack orders, then unpause the action. Making that sequence a one button control, like drinking a potion, would save a lot of play time and make combats run smoother.