you've got it wrong, i did waves in physics at the beggining of this year. two waves that arrive in the same place at the same time exactly in phase (same wave length & frequency) will constructivly 'superposition' which baisiclly means the frequency will double.
if two waves arrive in the same place at the same time exactly out of phase (for example one wave starts at the top & one starts at the bottom) it will destructivly superposition, canceling each other out.
here, this might be able to explain it a little better:
http://library.thinkquest.org/C00570...ves/phases.htm