Hi Sarah,
Unfortunately there is no way to break this down when creating the event and filling out the information under Event Details > Event Information.
I would suggest you insert the start time of the very first day of the event and the end time of the second day and make sure you have a clear agenda description and updates items for it in the Agenda page. You can personalize this under Website and Registration > Event Website > Pages.
You can edit the Instructions section within the Agenda page to make it clear for the invitees and registrants.
You can also make sure this is clear in the Invitation email and Registration Confirmation email.
Let us know if you have any questions!
Best,
Federica