When you register, you are registering for a spot in a specific run group.  If you would like to share a spot between two drivers, you may do that as long as you sign up for the run group that matches the experience level of the least experienced driver. 

Example:

Driver 1 is a beginner

Driver 2 is an intermediate level

Register for the beginner group.  When the beginner group is called, either driver may drive.  Complete one tech inspection form but add the contact information for both drivers.