Have you tried using the 'Merge Collection' action in the Utility - Collection Manipulation VBO for this to see if it meets your requirements? In your example, you may need to first split the columns in the first collection you wish to merge into the second collection, but it should be possible based on the action's description:
"This merges two collections with different structures (and where the field names across both collections are unique) and forms a single collection with a combination of the two structures and associated data values.
Note: This field will fail if both collections contain any field names which are identical."
