Difference between Swaha & Svadha

Daksha Prajapati has two daughters with name as Swaha (स्वाहा) & Svadha (स्वधा)

Swaha (स्वाहा) is wife of Agni Dev, therefore whenever an offering is made to Devata’s, than Swaha (स्वाहा) is pronounced. 

Svadha (स्वधा) is wife of Pitris (पितृ) or ancestors, therefore whenever an offering is made to pitris (पितृ), than Svadha (स्वधा) is pronounced.

As mentioned in  Srimad Bhagavat Purana,  Canto 4 Chapter 1 

स्वाहाभिमानिनश्चाग्नेरात्मजांस्त्रीनजीजनत्
पावकं पवमानं च शुचिं च हुतभोजनम् ।

Swaha (स्वाहा) is wife of Agni Dev, who gave birth to three children, named Pāvaka, Pavamāna and Śuchi, who existed on the oblations offered to Agni as sacrifice.

अग्निष्वात्ता बर्हिषद: सौम्या: पितर आज्यपा: ।
साग्नयोऽनग्नयस्तेषां पत्नी दाक्षायणी स्वधा ।

The Agniṣvāttas, the Barhiṣadas, the Saumyas and the Ājyapas are the
Pitris (पितृ).  The wife of all these Pitris is Svadha (स्वधा), who is the daughter of King Dakṣa.