No guarantee that this is the best way to do it, but:
1. Activate part2
2. Insert>Shared Geometry>Publish Geometry
3. Select all surfaces in part2 (select one surface, then Shift-select any other surface; should select all surfaces)
4. Go to assembly
5. Activate part1
6. Insert>Shared Geometry>Copy Geometry
7. Select the geometry you just published.
8. Edit>Solidify ... hit the "Remove Material" icon
That should do the trick. Geometry changes in part2 will update in part1-part2 upon regeneration.