i want to know if a duo can hold a relic and if they get it whos has it when they split

Yep, they can pick it up like any other character. When they split you decided who gets it.

What if two characters holding relics merge into a duo?

As a general rule, the duo would now have both relics. With the Book of the Skull specifically though it states that you can only double-equip hammers once 4 have been equipped, so if you don't have four equipped you can't Merge two people that have hammers, since the duo would then be double-equipped.

So if Titania, Wrecker and Absorbing Man have hammers, no Merge.

If Titania, Wrecker, Absorbing Man and Piledriver have hammers, Merge.

ok thank you