A life, or living in some difficulty? I personally think there are a lot of factors here. For example, are the twins old enough to make their own decisions? If so, and they consent, I believe that they should be allowed to be separated. Or, maybe they could live conjoined until they knew if they wanted the surgery or not. They'd be stared at and it would be incredibly difficult for them to live normal lives in conjoined, however is that worse then the chance of an innocent life being removed? Its hard to say.
It is ethical to attempt to separate conjoined twins even if there is a risk that one might die. It is not right to force two people to live as conjoined twins. Doctors should do everything in their power to make sure that at least one of the twins can have a normal life.
It is ethical to separate conjoined twins if there is a risk of one twin dying. I think they should be tried to be separated because without trying we will never know. It would not be worth the twins having to stay together all the time because it would not be a pleasant life for them.
A life joined to another in that unique way is something that should be avoided especially if the twins are young and are not yet used to this kind of life. It is compassionate to separate them even if it means giving one twin a full life and another dies in the process.
When twins are conjoined, parents should speak to doctors about the chances of each twin surviving if separated, as well as how well they will do if they remain conjoined. Only then can parents make the choice. I feel that if there is a high risk of one dying, but they would live a long life without being separated, then they should remain conjoined, as they can still live fulfilling lives. Abigail and Brittany Hensel are good examples of this.
I do not believe it is important to ask if it is ethical to separate conjoined twins if there is a risk of one twin dying. Cases of conjoined twins are rare, but information about them is easily available on the Internet. Each case is different. Surgery comes with the risk of death, that goes for a healthy person as well, so there are many other things to consider when deciding if conjoined twins should be separated. This is something the parents and the medical professional should discuss and decide on together. Each case is different, so you can't say it would be ethical/not ethical is all cases.