The two are being treated at the Princess Alexandra Hospital in Harlow.
One patient is said to be from west Essex and the other from the Hertfordshire area.
The patients were admitted for treatment at the weekend when tests confirmed the presence of the Legionella bacteria.
A team of experts in communicable disease control is investigating both cases to identify any links that could lead to the possible source of the infection, although no connection has yet been established.
Hospital staff and local doctors have been notified and asked to report any possible cases.
In a statement from the Essex Health Authority, officials said everything was being done to identify the cause and eliminate any risk of further infections.
Critical condition
A spokeswoman for the health authority said they were admitted to the hospital over the weekend and tests confirmed the Legionella infection.
She said: "They are in a critical but stable condition but there has been some improvement.
"A team of experts in communicable disease control is investigating both cases to identify any links that could lead to the possible source of the infection.
"No link has been identified and investigations are continuing."
Dr Sally Millership, consultant in communicable disease control at Essex Strategic Health Authority, said: "Isolated cases of Legionnaires' Disease do occur from time to time and it is possible that these two cases are coincidental.
"We are doing everything we can to identify the cause and eliminate any risk of further infections."