NP2 Tamamo or 2 Tamamos?

Because the title suggests you may not know, if you rolled 2 Tamamos the only thing you can really do is level her NP. You can't field 2 of the same servant from your own roster, you have to use a support.

That said, I'd field Tamamo + Waver unless you already have an attack buff from your mystic code or your dps, in which case you'd want double Tamamo. The reason being different damage buff types stack multiplicatively, while buffs of the same type only stack additively.