insert value into random row

I have a dataframe as below.

D1 = pd.DataFrame({'a': [15, 22, 107, 120],
                   'b': [25, 21, 95, 110]})

I am trying to randomly add two rows into column 'b' to get the effect of below. In each case the inserted 0 in this case shifts the rows down one.

D1 = pd.DataFrame({'a': [15, 22, 107, 120, 0, 0],
                   'b': [0, 25, 21, 0, 95, 110]})

Everything I have seen is about inserting into the whole column as opposed to individual rows.