Whats better dual wielding 1h crossbow or 1x 1h crossbow n a quiver n also on the calamity do i have to have to use marked for death skill for the bow ability to work?

Depends on build. UE: more people prefer bows than crossbow, but I guess its relative to what secondary (Disc) and base dmg.

If youre doing Nats, you'll be shat on from people for not having dual cross bow... nats / calamity|bale IIRC. Do you need MOD? No. Do you want it? Yes.

Im doing nats build so most people use nat crossbow n calamity but i think a quiver would be better or equivalent i just dont know which one to use

Quiver is not even viable for nats.

Mainhand nats slayer, then offhand best is balefire, then calamity if you don't have a decent balefire, or are running speed grift using a different element then fire.

I read somewhere that a quiver is only better for N6 if it's a 30%+ boost;/, so stick to the Calamity

No quiver can compete with the 130% crit damage a gem in offhand weapon gives you, regardless of how well it rolls. It would have to have a very good legendary effect to make up for it, and none of the current ones have effects that compliment nats enough to be worth it.