You can combine two arrays (with same length n) like this :
combined = [[array1], [array2]]
so that combined
is n x 2.
Although you can write your data without creating a third array:
openw, lun, 'path_to_file.ext', /get_lun
foreach elem, array1, index do begin
printf, lun, elem, array2[index]
endforeach
free_lun, lun