In Jain calendar terms, tithi means a lunar day.

• 1 tithi = one lunar date/day
• 2 tithi = two lunar dates/days

So the difference is simply the number of lunar days:

• 1 tithi is a single unit
• 2 tithi is two units, usually meaning the next tithi after the first

A tithi is not exactly the same as a 24-hour solar day. It changes according to the Moon’s position, so its length can vary.

If you want, I can also explain tithi in Jain fasting or Jain panchang in simple language.