Baldur's Gate II: Enhanced Edition

For the main quest "Rescue Adalon's Eggs" in Chapter 5, do not attempt to steal the eggs while your party is disguised as drows. Instead, work with Solaufein and complete his tasks. After you are sent to work with Phaere, complete her tasks, until she asks you to kill him. Your decision will affect which achievement you will unlock, at which point do one of the following:

Double Cross - If you kill Solaufein, then when you continue the quest, Phaere will eventually request that you switch the dragon eggs within the temple in Ust Natha with fakes. Agree to this, complete the task and bring the real eggs back to her. The achievement unlocks after accompanying her to the temple.

Triple Cross - If you spare Solaufein's life, the situation will remain the same as above, but with a big difference. After you agree to Phaere's request, Solaufein will request you give her fakes as well. Agree to this, and complete Phaere's task as before, but when you return to her, give her Solaufein's fakes. The achievement unlocks after accompanying her to the temple.

Both achievements can be acquired in a single playthrough by saving before completing one path, reloading, and then playing out the other path.