How to stack two columns in Google Sheet with column name in another column?

I have these columns in the Google Sheet.

Date        A   Date        B1/1/2020    3   5/2/2021    328.281/2/2020    41  5/3/2021    322.751/9/2020    23  5/25/2021   3421/12/2020   54  5/28/2021   3331/15/2020   1   7/1/2021    320.82

What I want is to concatenate these two columns vertically with dates and have the columns header on another column.

Date        Value   Name1/1/2020    3       A1/2/2020    41      A1/9/2020    23      A1/12/2020   54      A1/15/2020   1       A5/2/2021    328.28  B5/3/2021    322.75  B5/25/2021   342     B5/28/2021   333     B7/1/2021    320.82  B

more of like python's stack operation. ​But want in the Google Sheet for dynamic update. I am looking for an equation that can do that with N number of columns and N number of rows.