It is implemented for ePub , but there is no obvious menu entry.
On a KA1 change the default value in defaults.lua to 6. You can do this manually or via the advanced settings menu.
Code:
-- default minimum screen height for reading with 2 pages in landscape mode
DCREREADER_TWO_PAGE_THRESHOLD = 6
This will give you two columns in landscape mode. A value of 7 or higher will show one column.