Pandas – Interleave / Zip two DataFrames by row

You can sort the index after concatenating and then reset the index i.e

import pandas as pd

df1 = pd.DataFrame([['a','b','c'], ['d','e','f']])  
df2 = pd.DataFrame([['A','B','C'], ['D','E','F']])

concat_df = pd.concat([df1,df2]).sort_index().reset_index(drop=True)

Output :

   0  1  2
0  a  b  c
1  A  B  C
2  d  e  f
3  D  E  F

EDIT (OmerB) : Incase of keeping the order regardless of the index value then.

import pandas as pd
df1 = pd.DataFrame([['a','b','c'], ['d','e','f']]).reset_index()  
df2 = pd.DataFrame([['A','B','C'], ['D','E','F']]).reset_index()

concat_df = pd.concat([df1,df2]).sort_index().set_index('index')