What we call "magenta" is the sensation of both red and blue color-sensitive cells in the eye being excited at the same time. There's no single wavelength that produces this effect (unlike e.g. yellow). The closes you can get is violet, which looks faint to the eye.
A rainbow gives you both red and blue; mute everything else, and you'll get magenta. That's what magenta pigments do when illuminated by white light (which is a rainbow scrambled).
The interference is a wavelength too. Maybe not pure but it is one. Afaik they cannot be interpreted as two separate wavelengths and then “brain combined” when the aperture (the retina) is so small.
I haven't heard of a wavelength of 2 frequencies merged. It is like saying what is the wavelength if you tune to 2 radio stations with 2 radios (assume silent transmition for simplicity). There are 2 wavelengths.
Here's a nice visualization of color perception (there are more modern ones, but we used the 1931 color space when I was working in the field). The horseshoe shape on the outside is the single wavelength colors.