I would do the following
1) Create your FG surface in one Corridor Model
2) Create your Subgrade to top of Earthworks
3) Now create a surface offset from the key nodes at -2m for where your A1 Material Base would be if it existed.
4) Now set your corridor surface to be the A1 Base Material Layer (Corridor Properties - change the Material Layer)
5) Now use Merge Surfaces to merge the OG and the A1 Base Layer and use the Keep Maximum setting - this will keep the OG Model where it is above the A1 Layer and will keep the A1 Layer when it is above the OG
6) Now you can create the A1 Base Surface (the merged one) as a Surface Instruction in your corridor and apply the A1 Material above it
7) Set your Corridor Fill Material to be A2 Material (Corridor Properties or Edit Corridor).

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