Doesn't this assumption contradict the very fact that, in the additive system where light is concerned, magenta and yellow are secondary colors, which when combined, cannot form red (a primary additive), but rather form white?
Now that I have tried to confuse everyone with the previous post, I see that I didn't read your questions completely.

In your example, I think that actually the result WILL BE reddish light, not white. Here's how: you want to add (not filter) lights, using magenta light and yellow light. In terms of red, green, and blue, the magenta light contains red and blue (it is missing green light). Similarly, the yellow light contains red and green (it is missing blue light). So combining these, we get (red and blue) plus (red and green), in other words, a double portion of red light, plus a single portion of green light and a single portion of blue light. so the overall result should have a distinctly reddish appearance. Of course, it is not a pure red light - it has been contaminated with some green and some blue, but reddish nonetheless.

I hope this is some help, rather than just confusing.