It is possible for two people to have the same number on the arrest warrant.  A best practice is to have them on separate warrants, however, it will not invalidate the warrant to have both defendants listed in the same warrant.  Regardless, in order to be valid for both defendants, there must be sufficient evidence to support the arrest of both defendants.  If the facts are insufficient for one of the defendants, then the warrant can be thrown out in the case where the facts are insufficient to support the arrest.