They were elected to the Lok Sabha on the Samata Party's symbol after they merged their SJP with the George Fernandes-led Samata Party on the eve of the elections.
They refused to go along with Fernandes' alliance with the BJP after the elections.
Ram Bahadur Singh, MP from Bihar, and Bhakta Charan Das, from Orissa, are the two members who have written to Sangma along with Chandrashekhar.
SP general secretary SP Malviya said the Election Commission had accepted the SJP as an unrecognised party following a request. He said the party's symbol for the assembly elections in Jammu and Kashmir and Uttar Pradesh and for the by-elections in Bihar would be the umbrella.