I've often seen the names, Lord Ayyappa and Sri Dharma Sastha. Lord Ayyappa is the Son of Lord Shiva and Lord Vishnu (Mohini).But what are the differences between them? Are Lord Ayyappa and Sri Dharma Sastha the same?
Theoritically they are different.
Ayyappan was the adopted son of the king of pandalam, actually the son of Shiva and Mohini. He was a Nitya Bramhachari. (https://en.wikipedia.org/wiki/Ayyappan)
But Dharma Sastha means a man who has the power to make and implement dharmas or righteousness in the society. He also has 2 wives. (http://vipasana-vidushika.blogspot.in/2013/05/ayyappa-as-dharma-sastha.html)
The problem arises because of Sabarimala Ayyappa temple being named and also conducting poojas on Dharma Sastha
This is a kind of question which should be answered by the yogis or those who are in those lines, those who converse with the divine power, rather than scholars who read books which itself is many times altered and wild and creative imagination for a noble purpose. As per the tantrik beliefs, both are different, one born as a human, and the other is siva-narayana chaithanya. The truth is more bitter, so better, be with the Sastha form. So, in temple worships, Sastha form only has to be worshiped, as this can often lead to many issues, those who know this, will keep this with them, to avoid confusion to common lot.