Merge/append two dataframes with common and different columns

Question

I'm trying to combine two dataframes in Python. Each has unique columns and common columns. The Key column is the same for both files.

In the combined dataframe, I want to keep each key as two rows (1 for each timepoint) and I want values in added columns to be filled with NaN if this column was not in original timepoint

  Example

CSV 1:
Key   Make   Model  Timepoint
501   Audi   A3.       1
502   Audi   NaN       1
503   Audi   A5.       1

CSV 2:
Key   Make   Model  Engine  Timepoint
501   Audi   A3      2.0T      2
502   BMW    A4      5.6T      2
503   Audi   A2      1.4T      2


Combined Expected Result:
Key   Make   Model   Engine  Timepoint
501   Audi    A3       NaN       1
502   Audi    NaN      NaN       1
503   Audi    A5       NaN       1
501  Audi     A3       2.0T      2
502   BMW     A4       5.6T      2
503   Audi    A2       1.4T      2