If the filter is a dichroic filter, it is an interference filter that won't fade.
They do, however, become less effective if they become dirty.
While dirt on a filter might cut down on the amount of light there should be no change in its spectral response.
Dug around your earlier posts where you say the maximum for a Chromega is 170 units, and you do split-grade printing.
So burying an additional Magenta inside the housing would become a chore. I'd put a 30-50 Magenta filter in front of the lens so you could go up past 200 if you wanted to.
There is a basic difference in the way dichroic heads and filter drawers use their filters. A filter in a drawer filters all of the light while a dichro head(subtractive w/moving filters) filters some of the light till you get to the maximum position, leaving some of the light unfiltered most of the time.
A second magenta would in fact reduce the light for the high contrast layer, rebalancing the output toward the low. You would need to use a separate filter to get rid of the light that goes past the edge of the moving filter. Not much- a 40m in the drawer would let you get all the contrast available when added to a fully magenta blocked aperture.
Ilford under-lens filter kit, with white-light from the enlarger?? Works for me anyway, and I made holders from foamboard that allow easy lens changes (the holder that comes with the set, mounts on the lens thread).
Not to make things more confusing, but I have the same enlarger and head, and I find the Dichroic filters can be a bit short on maximum contrast. My serrendipitous solution was to get an old set of dupont or kodak under the lens filters, with bakelite screw on the lens holder. The number 5 there gives a contrastier result.
On the assumption your dichroic filtes act as perfect filters, doubling up on the same filter will only let the same image developing wavelengths pass twice, but will make it harder for your eyes to see the image on the easil.