For the life of me I can't remember where I got this direction from, but I just updated much of this set that has two numbers per card. I did it after reading that the cards should be "# / #". Don't see why the same wouldn't apply here.
Edit: Ahh...it was in the "Revised Rules and Guidelines" PDF.
Guideline 10E—Some cards, or more commonly stickers, are issued as multiples on standard sized stock. When the separate images on these cards have individual but non-sequential numbers (e.g., 144 & 162),
the number should be listed as "144 / 162", with no additional info in the Note field. If the images are sequentially numbered (e.g., 7, 8, 9), a dash is used and the number is listed as "7-9"