Since you have 2 sepearate criterias you are going to ahve to use 2 seperate INSERT statements. Your first statement will consist of an INSERT where the departure date is 1 day greater then the arrival date. The 2nd statement would be INSERT where the departure date is 2 days greater then the arrival date. It's hard for me to write code without knowing your tables but hopefully the example below shows it:

INSERT INTO NewTable SELECT Count(CustomerID) as CountOf1DayAfter FROM OriginalTable WHERE (DepartureDate - ArrivalDate) = 1

INSERT INTO NewTable SELECT Count(CustomerID) as CountOf2DaysAfter FROM OriginalTable WHERE (DepartureDate - ArrivalDate) = 2