I created another column, column C, with the start time + two hours formula:
Code:
=IF(A1 <> "", A1 + TIME(2,0,0), "")
Then I hid the column. Then set the data validation (list) for the end time column (column B) to this formula:
Code:
=OFFSET(INDIRECT(ADDRESS(ROW(), COLUMN())),0,1,1,1)
Which gives a drop-down list that contains only the contents of the cell one column to the right (which contains the start time + two hours).