How do I append and update overlapping data as new versions come in?

Let's say I have these two sets of data in Excel files:

Each covers two weeks of daily data with one overlapping week. I'd like to blend the two sets so that I have Week 1 from the Set 1, Week 3 from the Set 2, and Week 2 from Set 2 as it is the most recent version of that overlapping week. I think of this as appending and updating data.

If I do a straight Union, the result has two entries for each of the overlapping days. 

How can I alter the Union or use another tool to append and update data on an ongoing basis?

Thanks!