If I want to list only fonts that have fontformat=TrueType or fontformat=CFF, currently I can't:

behdad:~ 0$ fc-list :fontformat=truetype,cff

Returns empty.

This is because FcFontList interprets that as returning fonts that have both fontformat=truetype AND fontformat=cff.  Code:

/*
 * Font must have a containing value for every value in the pattern
 */
static FcBool
FcListValueListMatchAny (FcValueListPtr patOrig,            /* pattern */
                         FcValueListPtr fntOrig)            /* font */
{
    FcValueListPtr       pat, fnt;

    for (pat = patOrig; pat != NULL; pat = FcValueListNext(pat))
    {
        for (fnt = fntOrig; fnt != NULL; fnt = FcValueListNext(fnt))
        {
            /*
             * make sure the font 'contains' the pattern.
             * (OpListing is OpContains except for strings
             *  where it requires an exact match)
             */
            if (FcConfigCompareValue (&fnt->value,
                                      FC_OP (FcOpListing, FcOpFlagIgnoreBlanks),
                                      &pat->value))
                break;
        }
        if (fnt == NULL)
            return FcFalse;
    }
    return FcTrue;
}

I cannot imagine any scenario that this semantic is useful.  I suggest we flip it from an AND to an OR.