You select two BOM headers in the BOM Headers overview and switch to the Items overview. In the context you first of all see the data for the first BOM header. In the overview you see its items.

The "(1/2)" indicator signifies that the Items overview is currently displaying the items for the first of the two selected bills of material (see graphic). If you display the items for the second selected bill of material the indicator switches to "(2/2)".

Since you have made selections on the BOM headers overview, you can make entries in the lines for material and BOM header.