There's no way to know the correct cumulative filter factor of sandwiching filters without actual testing per SPECIFIC film. I gave a reasonable hunch per TMax films, which have a little more sensitivity to green than Ilford films, but a little less sensitivity to blue.
As far as just buying Ortho 80 instead, that's not going to be practical if one wants to turn around and take a regular pan film shots alternately too, or when shooting large sizes of expensive sheet film.
Of course, if one wishes to predominantly take only Ortho shots, might as well buy a box of the real deal. But I happen to sometimes like the look or old blue-only sensitive emulsions instead, which calls for a deep blue 47 filter with pan film.
If you tried that with Ortho 80, you'd probably be down to an ASA of only 10 !
Then there are specific grain and curve shape issues which dictate the film choice, so mandate a malleable pan emulsion choice. It's all fun, regardless.