If by dassa you mean ekasana / dasana and by bissa you mean biyasana, then the difference is:

• Dassa / Ekasana: taking food once a day in one sitting.
• Bissa / Biyasana: taking food twice a day in two separate sittings. ( jainknowledge.com)

So, ekasana is stricter than biyasana. Both are Jain dietary disciplines meant to increase self-control and reduce attachment to food. ( jainknowledge.com)

If you want, I can also explain the difference in Jain fasting terms like ekasana, biyasana, ayambil, upvas, and chauvihar.