The bosses (Vic Vox and Vinny Vox) would drop one of the following: Leadership's Price Onyx Amulet.
PoE 3.12.4: GGG lowers the Drop Level of The Twins Unique Contract from Level 83 to Level 81 to make it slightly more accessible. These changes aren't in patch 3.12.4 but will be deployed as soon as possible.
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How to get Contract: The Twins?
Farm Location: Reward Room Chests in Heists and Grand Heists.
PoE 3.12.4b: currently, the only way to farm unique Contract: The Twins is through completing the appropriate questlines and then hoping that they drop from Smuggler's Caches within maps. GGG has seen feedback that the drop rate is too low and that for players that are predominantly running Heist content, they're not seeing any. To mitigate this, GGG is making it so that Unique Contracts can also drop from Reward Room Chests in Heists. This greatly increases their drop rate overall for players who are running both map content and Heist content.
Contract: The Twins Guide
PoE: Challenge Conditionally Explained--Contract: The Twins Vengeful Contract