Harmonic balance v Envelope simulation for two tone test

Hi all. I was wondering how exactly two tone (or multi-tone) simulations are performed in harmonic balance? I have found the literature to explain the single tone case but becomes ambiguous when multi-tone excitation is discussed. As a two-tone excitation involves a time varying envelope, I am curious as how harmonic balance deals with this? Also, will circuit envelope simulations provide a more accurate result for a two-tone test as it can deal with time varying envelopes? Thanks.