Mark,
I don't use AddItem, normally I use an array property. But generating a quick form, dropping a lst on it, setting ColumnCount=2, ColumnWitdh=100,100 then running the form and playing in the command window:
x=lstaddlistitem.list1
x.additem( "col1, row1", 1 )
x.addlistitem( "col2, row1", 1, 2 )
x.additem( "col1, row2", 2 )
x.addlistitem( "col2, row2", 2, 2 )
>That was quick! But ..
>ThisForm.LstList.AddListItem('123', 1)
>ThisForm.LstList.AddListItem('456', 2)
>still puts data into the first column only.