Go to the admission item that they are associated with and set it from there.
Registration > Admission Item > Advanced Settings
Make sure they are both associated with the admission item and right under that section is the 'Rules for Optional Sessions' section. Select a minimum of 1. You could also set a maximum of 2 if you wanted but it's not necessary since there are only two options to begin with. If you only wanted them to be able to select 1 of the sessions, then you'd set both the min and max to 1.
------------------------------
Sara Shidal
Computer Services, Inc
------------------------------