You can force legend to create multiple columns by selecting 'Place in new column' for layer which should be in new column and leaving number of columns=1.
Look at this picture:
Only the FC4 is selected to 'Place in new column' and number of columns in 'Columns:' field is left default (1) for each layer.
All the layers below FC4 (in 'Legend Items:' list) are placed in this new column.
If you for example addictionaly select FC7 to 'Place in new column', there will be 3 columns in legend: 1: FC1,FC2,FC3; 2: FC4,FC5,FC6; 3: FC7,FC8.
You can achieve this same effect by setting numbers of columns in 'Columns:' field to 2 for FC4 and leaving 'Place in new column' unselected: