so, put simply i believe we should be able to combine these 2 items for 2 very simple reasons.
1. more than 1 type of ammo in quiver. 2. more ammo being able to be held than bolt pouch provides.
currently bolt pouch has 4 slots which each hold 255 bolts of any type. meaning u can only carry 1,020 total bolts, making it only useful for when mobs drop more than 1 type of bolt. allowing you to save inventory space as long as u don't go over the limit.