Alternate jurors must be drawn from the same source and in the same manner and have the same qualifications as the jurors already sworn. They shall be subject to the same examination and challenges. The state shall be entitled to as many peremptory challenges to alternate jurors as there are alternate jurors called. The defendant shall be entitled to additional peremptory challenges in an amount twice greater than the additional peremptory challenges of the state. The peremptory challenges allowed to the state and to the defendant in such event shall be in addition to the regular number of peremptory challenges allowed in criminal cases to the defendant and to the state as provided by law. When two or more defendants are tried jointly, each defendant shall be entitled to as many peremptory challenges to alternate jurors as there are alternate jurors called.