For warm days, just using yellow wears off really fast.
Yes. It does. If that's the substance of your waxing challenge, I don't think anyone's going to have a GREAT solution. The one from @cantunamunch
is the closest. If your most recent layer is a high-fluoro warm wax, so much the better (for your skiing, if not for your health).
Certainly MIXING two kinds of wax isn't going to help much because when it's really warm you want the warmest wax you can get.
As for whether you can delay the inevitable by covering the warm wax with a layer of colder wax, I don't know. Even if it works, I'd think the timing would be hard to get right.
Regardless, once the warm wax wears off, you've just got to put more on. Rub-on waxes are your friend here.