In Jain Dharma, two Vandana is generally done to show extra humility, reverence, and deep respect.

It is used especially:

• to honor the Guru Bhagwan / Sadhu-Sadhvi
• to express vinay and bhakti
• to seek forgiveness for mistakes in conduct, speech, or thought
• to purify the mind before or during pratikraman or other devotional acts

So, the specific reason is deep respectful salutation with humility and repentance.

In short: Two Vandana means stronger vinay and bhavna of respect.