Because some other element may have a line at the same wavelength. So we use TWO lines minimum since an interfering element USUALLY doesn't have a second interfering line. In saying this however, bear in mind that element A may be interfered with at the primary line with element B while the second line for A may be interfered with by element C or D or some other element. There are two things to do. One is to use more than two lines for the identification process; the other is to use a set of tables that uses lines for identification that are virtually interference free. Such tables exist.
