Answer

No. Chaat cannot be called 2 Atthams.

In Jain tapasya, Attham has a specific meaning: fasting continuously for 3 days, usually with only boiled water during permitted time, according to one’s tradition.

So:

• 1 Attham = 3 days of upvas
• 2 Atthams = 6 days of upvas

Chhath / Chhath tap usually refers to 2 days of upvas, not 2 Atthams.

Takeaway

If someone has done 2 days fast, it should be called Chhath, not 2 Atthams. 2 Atthams means 6 days fasting.