Without knowing the exact catalog number, the general editorial approach that Scott uses even if it doesn't assign a major/minor catalog number to perforation varieties, is to list the known perforations in the header.
If the header says "Perf 11, 14" then it means the stamps below are found with both perforation measurements. Normally, if compound perforations exist, it will add it or say something like "Perf. 11, 14, and compound" to indicate that perf 11x14 and/or perf 14x11 exist, depending on the wording. For example, see Scott Hungary B80-B87(1925) athletics association set. But as WERT noted, knowing the specific Scott number would be helpful.