The Communist Party of India-Marxist won from Pratapgarh for the ninth consecutive time and won in Surma for the eighth time in a row, officials said.
CPI-M state secretary and central committee member Bijan Dhar told IANS: "The voters supported the CPI-M due to the good governance of the Left Front. Our victory in Tripura will continue in future elections too."
In Pratapgarh, Ramu Das defeated his Bharatiya Janata Party rival Mousumi Das by 17,326 votes. The CPI-M secured 27,555 votes, the BJP 10,229 votes while Congress' Ranjit Kumar Das bagged only 5,187 votes.
In Surma, CPI-M contender Anjan Das routed BJP's Ashish Das by 15,309 votes. The CPI-M got 23,275 votes, the BJP 7,966 votes and Nagendra Chandra Biswas of Congress bagged 2,528 votes.
The results appeared to show a major dent in Congress support in Tripura, with its traditional votes getting divided between the Left Front and a resurgent BJP.
Reacting to its increased vote share in both assembly seats, BJP state president Sudhindra Dasgupta told IANS: "The BJP will be the main opposition in Tripura now and the party will be successful in the next assembly elections in 2018."
Both seats were reserved for the Scheduled Caste.
In February 2013, CPI-M candidates won by 2,132 votes from Pratapgarh and by 1,862 votes in Surma.
The by-elections were necessitated after the death of CPI-M stalwart and former minister Anil Sarkar (Pratapgarh) and former minister Sudhir Das (Surma).