Steps:
1. Load DSO 1 data and DSO 2 data
2. Load data from DSO 1 only via a Transformation to Final DSO
3. (A) If using OOPS ABAP - Create a Method in Class to lookup remaining fields using data from DSO 2, while loading data from DSO 1.
(B) If not using OOPS: Write code in Start Routine directly.
For Step 3 above, as per your example, comparison can be done using "Material Document" and "Ref Key".
Thanks!!!