I initially thought that even with a different amplitude two waves will still form a standing wave if they have the same frequency and move in opposite directions. So I tested it using Mr Loo Wee Kang's simulation and it turned out I was wrong. The blue and green waves are the individual waves moving in opposite directions. The red is vector sum of the two waves. As observed from the video, the red wave is not a standing wave but propagates.
For my past, present and future students.