Hi Maggie,
Do you have a limit set for how many sessions that registrants are able to select in the registration process? If so, within the advanced settings of your admission item(s) you can set a maximum number of sessions that each invitee can select and you have the option to "count waitlisted sessions towards these limits" yes or no. If you set this to no, they will be able to add themselves to the waitlist for a session and then also select another session. See screenshot below as an example:

------------------------------
Julia Plymack
Senior Customer Success Manager
Cvent
------------------------------