It is now possible to "split" a multi-day booking into two! (since 6.6)
It's super easy... simply highlight the day or days (in Hourline click at date on top, or Long Form) you want the booking to be split on > click on booking menu > split > confirm.
* Remember not to select the first day of the booking!
Result is two or more bookings - one from the start of the original booking to the day before the highlighted day, the other from the highlighted day to the end of the booking (booking name, status, custom fields, involved objects, etc - all remain identical for both bookings).