u/Icarusqt

Or can I wear two of the same ring?

On paper, a third slot would give you a better chance. But if I'm correct, you can only get loot from the bosses you killed in raid in your Vault. So getting that 3rd slot also adds 2 extra boss's loot pools to your slots. I'm sure it's close, but there's gotta be a mathematically percent to which is actually better.