I think it is reasonable to allow for both to be installed. Some graphics applications make better use of OTF vs TTF, and vise-versa. Given the font is niche enough, I would like to think people that choose one or the other may be able to understand what best works for their usecase.
We could probably do with patterning TTF files into font-firago-ttf and doing a replaces of font-firago to font-firago-ttf. We can keep the same "main" (so not renaming the name in package.yml itself), we just aren't producing a package for it, similar to what we do for LibreOffice.