More than one user cannot have a cell open for edit at the same time. To do so would be a potential disaster. Merging GDS is also a danger because one' fix may conflict with another's fix.
That said, more than one can work on the same layout at the same time as long as they only open a cell for edit while they are working on it. Using Edit In Place and Save and Make Read Only will make this work:
1. Each designer opens their own empty cellView.
2. Place the top level cell with the error cells as an instance in the cellView
3. Use edit-in-place to push into the cells to make the edits. If all the edits are in the top level cell, this won't work but often they are in lower level cells as well.
4. After editing, the user Saves the current cell, checks it in if using design management, makes it read only and returns to the top.
If all the edits in the top level cell, you may have to split the data in the top level cell into multiple cells (quadrant cells?) so each user can edit on a different cell to fix the errors.