Two "total number of participants"?
Since you have a different participant limit, its really a distinct event and you should treat it that way by creating a second event.  Yes your participants have to register twice but then you'll have a much nicer record of which day they attended (and all the benefits of automated reminders as well).

With the two-event configuration you can offer registrants of the first event a special discount for the second - just include the discount code in the confirmation email. You'll need to configure your discount codes carefully to activate and de-activate at the correct time. As a bonus, include a checksum link to the registration page, then the registrant won't have to re-enter all their information.