Conjoined twins have been born in Brazil with two heads, two functioning brains and two backbones - but a single heart.
The rare condition is thought to have occurred when one of the pair failed to fully develop in the womb.
Doctors say separating the twins, named Jesus and Emanuel, is not currently an option because there is only one set of organs, Reuters reports.
They are being monitored by specialists to see how they develop.
Dr Neila Dahas, who is treating the newborns, said surgery was not being considered at the moment.
But she said separating the boys would be impossible because of the single set of organs - and that it was difficult to choose what to do because both brains were functioning well.
"What we know statistically is that the children who undergo surgery and survive are the children who have less organs in common," she added.
"What we've got to think about at this moment is to maintain the children in good condition and see how they will develop."
The condition, known as dicephalic parapagus, is rare.
However there have been other known cases, notably Abigail and Brittany Hensel who were born in the US in 1990. They aim to live as normal a life as possible, even taking their driving test when they were 16.
Jesus and Emanuel were born by Caesarean section weighing 9.9lbs (4.5kg) on Monday morning in a small hospital in the northern state of Para.
They were then taken by plane to a better equipped hospital in the state capital Belem.
Doctors say the mother breastfed both boys a few times and that their appetite is normal.
Claudioner Assis de Vasconcelos, director of the hospital in Anajas where she gave birth, told Brazil's O Povo newspaper that she came in because she was experiencing strong abdominal pains.
It is reported that the 25-year-old, who lives in a remote area, did not have any ultrasound scans during her pregnancy - and only found out about her sons' condition minutes before the birth.
Mr de Vasconcelos said: "Despite all the problems we have as a small interior hospital we managed to save both mother and baby, which was our aim."
Patrick O'Brien, a spokesman for the UK's Royal College of Obstetrician and Gynaecologists who has been involved in several conjoined twin cases, said no decisions were likely to be made about Jesus and Emanuel's future for some time.
"A lot of work is needed, in terms of scans and tests, before doctors will know if they can separate them or not, and just how organs and blood vessels are shared and linked.
"It takes quite a while before they can decide how feasible it is."
Mr O'Brien said dicephalic parapagus affected around one in 100,000 pregnancies, but that around half do not reach full-term.