The two eye views are almost the same image. How do you avoid paying for the scene twice?

Short answer

Only the view and projection matrices differ between eyes, so scene traversal, culling, animation and shadow maps can be done once, and multiview rendering lets one draw call emit both views with a per-view matrix chosen in the vertex shader.

Keep stereo rendering explicit in the answer because that is the concept the interviewer is actually trying to test. A good stereo rendering explanation names the trade-off, the failure mode, and the evidence you would use before choosing. Use stereo rendering once more at the decision point so the answer reads as judgement rather than a detached example.

Keep stereo rendering explicit in the answer because that is the concept the interviewer is actually trying to test. A good stereo rendering explanation names the trade-off, the failure mode, and the evidence you would use before choosing.