That's such a specific scenario that I actually love. The main conflict I see isn't with the outside world—it's between them. If they both remember a past life where, say, Twin A died protecting Twin B, the survivor's guilt could be suffocating. The 'protector' twin might become overbearing, trying to control every step to prevent history from repeating, while the 'protected' twin could rebel fiercely against that smothering care, creating a horrible rift.
Their shared knowledge is a double-edged sword. Sure, they can plan and manipulate events together, but it also means they can't hide their true selves from each other. There's no private space for personal grief or trauma from the past life. Every nightmare, every flinch, gets witnessed and analyzed. The pressure to present a united, flawless front to the family they've returned to might crack them under the weight of their secret.
Frankly, the most chilling challenge might be realizing their goals have diverged. What if one twin wants peaceful obscurity this time, and the other is burning for revenge? That shared past becomes the battlefield.