why would you ever buy two Bloodthirsters though?
On any ADC, never mind Twitch.
Let's take Twitch for example. One Bloodthirster on Twitch is not a guarantee, never mind two. These are in no particular order btw.
You want Blade of the Ruined King for the attack speed, lifesteal and the active as it gives you a tonne of pick potential, as well as the passive % HP damage.
You want a Phantom Dancer/Statikk Shiv for the attack speed, crit chance and movement speed.
You want a IE for the crit damage and crit chance as well as a decent amount of raw AD as it ups your DPS massively, especially when you're ulting an entire team.
You want a Last Whisper for the massive armour penetration. Again, necessary.
You need boots for the mobility and attack speed, as well as the utility boot enchantments give.
Then you have one item slot left. It's here where you could fit in one BT, but you probably want a defensive item because Twitch is an immobile, vulnerable ADC.
I don't see where you're ever fitting in a second Bloodthirster there. The raw AD is not worth it over the necessary stats from other items, not to mention as Leezard says if you die, that's a tonne of your power and that item's value lost until you can farm it again.
In terms of Draven, you want BT, PD, Last Whisper, IE, Boots and a defensive item, because again you're immobile and vulnerable. You should never be buying two BTs.
