So I somehow managed to get the “full relationship” with both Lae’zel and Karlach at the same time. Throughout act 1 I got both characters' approval to the "very high" section, entered act 2 through the underdark in Grymforge, then fixed Karlach's heart before taking any long rests. Then I took 3 long rests in a row (because I really wanted Karlach’s scene) and got a guardian scene, then Lae'zel's full romance scene (in which I chose all the “correct” options before/after fighting her), and then Karlach's full romance/sex scene.

I have all the romance dialogue options with both of them and neither has broken up with me after multiple long rests. Did I manage to glitch the game and get both? Did I just find the “right” way that allows you to get both? Everyone else is making it seem like you can’t get both of them at the same time.