You use the white scroll before using the primary scroll. The white scroll will save a slot if you fail a scroll. If the scroll is a success, then the white scroll is still used and the slot is used up.
Also, this glitch has been fixed, but if you use your white scroll with a clean slate, cape warmth, or shoe spike, and the scroll fails, the white scroll was not used up AND you actually gained a slot. +30 100%ed pac anyone~?