So can I cry that is unfair for me as a BRD to only have one bow and I can't equip another thing in my off-hand, like stronger arrows or something (like diablo)?

You have two items, therefore two infusing.
IF the item were Curtana Sword/Holy Shield, as a pack (therefore, no off-hand stuff, just 'weapon set'), you would have to make only one. But then you couldn't have different shields