df = pd.DataFrame([[1,2,3,4],[5,6,7,8],[9,10,11,12]], columns = ['A', 'B', 'A1', 'B1'])
dfA = df[['A','B']]
dfB = df[['A1','B1']]
dfB = dfB.rename(columns = {'A1':'A', 'B1':'B'})
dfA['id'] = 1
dfB['id'] = 2
dfC = pd.concat([dfA, dfB])