Short answer: No. Chauvihaar upvas is not equivalent to two Tivihar upvas.

Why:

• Chauvihaar Upvas: complete abstinence from all food and water from sunset to the next sunrise (roughly a 24–36 hour period, depending on local practice). It is stricter because no water is taken at all.
• Tivihar Upvas: abstain from solid food for the day, but boiled water is allowed during the daylight hours; some practices allow water after sunrise but before sunset.

Because Tivihar allows water during the day and Chauvihaar forbids water, they are different austerities. There isn’t a standard rule that two Tivihar fasts equal one Chauvihaar; Chauvihaar remains the stricter, more austere practice. You can read more here