Prerequisities to become an MD-PhD?
Most of the "legal requirements" associated with MD-PhD programs are financing related. In general, such programs are usually sponsored and funded by national health ministries (or their equivalents). Therefore, it is preferred (or required) only to have legally qualified citizens participate in such programs. 

As for the choice of major, in general, computer science is not one of the usual entry points for MD-PhD programs, which tend to be more closely related to experimental programs and majors (e.g., chemistry, biochemistry, biology, bioengineering). That said, there is more of an emphasis nowadays on computational science in medicine, and more and more MD-PhD programs are expanding their scope to include bioinformatics and computational biology. But it is still definitely not a universal, and CS majors will be at something of a disadvantage.