Conjoined twins are legally classified as two separate persons. Although both conjoined, each will have their own birth certificate and driving licence, for example. The Hensen twins took their driving test separately, even though they both share the use of two legs and two arms.
In that way, they are each criminally liable for their own actions.
It does however, raise the question over what would happen if they committed a driving offence.
In their case, the person liable would be whichever one was controlling the driving at the time.
The same principle would apply, for example, if they killed someone by kicking and punching someone to death. In their case, they each have control of one leg and one arm, and co operate with each other to use them to accomplish any task.
An interesting question would occur if they picked up a gun and shot someone.
In this case, I assume that the person who had control of the gun hand would be responsible.
So to conclude, I would suggest that each circumstance would be different , and would depend entirely on how two individuals are conjoined.
In the case where two people were conjoined only at the hip, each with control of their own limbs, it would be much easier to determine whoever was responsible for a crime, for example, as in the Hilton twins who each had independent control of their own bodies and shared no major organs.
A crime committed by one conjoined twin does not automatically mean that the non-perpetrator is an accessory. Mere presence is not enough – generally, an accessory must intentionally assist or encourage the offender, and must promote or assist in the crime.
Sentencing, however, would present a particular conundrum.
If you determine whichever twin committed a crime which attracted a prison sentence, then how can you send them to prison- because you would certainly be putting at least one innocent person in jail.
To answer that question would require precedent, and unfortunately, there isn’t any.