Merging Pandas Dataframes averiging values where both have values

Question

I have two Dataframes.

print(df1)
  key  value
0   A  NaN
1   B  1
2   C  NaN
3   D  3

print(df2)
  key  value
0   B  NaN
1   D  3
2   E  1
3   F  NaN