Compoundoku wants REALLY BIG!

Rules of Compoundoku:

• Solve both left and right Sudokus.
• In addition, the board below them is the Compound Board of both Sudokus.
• Each number on the Compound Board should tell either: (1) the number on the left Sudoku, or (2) the sum of both numbers on the left and right Sudokus; in the respective position.

^{Credit to @athin for the original idea of this sudoku variant.
The original and ‘BIG’ version can be found here and here.}