Height differences are always challenging. Think about what lens you're on, and figure out the distance you need to be from both actors/actresses to maintain your desired composition. Also think about being able to hold them if you're going around corners, and whether or not losing sight of them for a few moments is okay or not - a good conversation to have with DP and/or director.
A tight two shot typically isn't very aesthetically pleasing if one head is at the bottom of frame and one is at the top, or your whole frame is dutched to get both, but as with everything else, it depends on the story you're trying to tell. Maybe low mode is a better choice because you see more of the upper wall behind them and notice a picture on the wall that's important to the story that you'd miss in high mode. Experiment and see what feels right to you for what you're trying to convey in the shot.