Python:concat两个不同形状的数据帧的行

我有两个数据帧:

df1 = pd.DataFrame(index = [0,1,2], columns=['USD', 'CAD'])
df1['USD'] = [1,2,3]
df1['CAD'] = [4,5,6]
df1:
    USD CAD
0   1   4
1   2   5
2   3   6

df2 = pd.DataFrame(index = [0,1], columns = ['currency','balance'])
df2['currency'] = ['USD', 'CAD']
df2['balance'] = [2,3]
df2:
    currency    balance
0   USD         2
1   CAD         3

我想在df1的索引0处添加一行,并在该行中填入基于货币的df2余额。最后的df应该是这样的:

df:
    USD CAD
0   2   3
1   1   4
2   2   5
3   3   6

我怎么能用蟒蛇的方式来做呢?非常感谢。