Example: "I have a single guest in a double room. Now I want to add a second guest to the same unit/room for some days during the same stay."
1) First shorten the check in and check out in the booking corresponding to the number of nights for the first guest only.
2)Then click on "Add accommodation" in the booking. Choose same unit, choose check in date for the period for two persons stay. Choose two persons. Click ok. Now two rows is created in the booking for the same unit. One holds the single stay period, and one holds the 2 guest stay period, with updated price.
Observe: When attempting to "add accommodation" in the same booking it would not let you use a check-in date after the original check-in date, only before/older if you not first choose the checkout date before check in date!