When you add 2 dyes to 1 emulsion, the more tightly adsorbed one can kick the other dye off the grain and you lose sensitivity in one region of the spectrum. That is why I suggest mixing 2 differently sensitized emulsions.
As for a pan dye, well that is the problem. I have not yet found one commercially. We had them at EK. There is a problem with these too. Their spectral distribution may not match the spectrum you wish to capture, and it may vary from emulsion to emulsion. That is another reason I prefer the blending method. Of course this is only a problem with B&W materials. I've been fortunate that all of my work with spectral sensitization was in color where it was mandatory that a minimum of 3 emulsions be used. The B&W work was purely "theoretical" in meetings and such reviewing the work of the B&W people.