dataTables (jquery) search by two columns

I want to search table by only two columns. E.g. Initial search searches in all columns, I just want to search by two columns. When I use following code:

dataTable.columns(1,7)
    .search("some string")
    .draw(); 

the search text must be present in both columns to be displayed. I want to display it even if it exists in one of the columns. Can anyone help?

Thanks