The deceased were identified as Mohammad Ali, 34, son of Amzad Ali, and construction worker Naim Hossain, son of Nazrul Islam.
Quoting locals, police said Naim went inside the under-construction septic tank of the house of Mohammad Ali around 10:30am for removing bamboo poles. However, he fell unconscious inside the septic tank.
Getting no response from Naim, Mohammad Ali also went down the tank and fell unconscious, said Mansur Rahman, officer-in-charge of Panchbibi Police Station.
Zakaria Hossain, 26, a neighbour of Mohammad Ali, also fell sick while he went to save the duo, the OC said.
On information, a team of Fire Service and Civil Defence went to the spot and rescued them. Later, Naim and Mohammad were taken to Modern District Hospital where doctors declared them dead.
Zakaria was undergoing treatment at a local hospital.