It all depends on what you mean by affect. Two em waves in the same space will have a different overall amplitude at any frequency.
If you mean as in the overall color of light, that will change based on how much/what frequency waves are combined. Think about adding a bit of black sand to a jar of white sand – from a distance it will appear grey but the actual colors of individual grains of sand (frequency of “individual” em waves) won’t change.
For wifi, data transmission is via phase modulation of the em wave, so the signal is resilient against adding different frequencies/amplitudes but may suffer if the same frequency is transmitted at a different phase.