Is an operator applied to cloned geometries, which in turn replicates all actions applied to the original mesh simultaneously. This would save you a lot of time, since you won’t have to go in and edit the clones one by one.
Embedded below, is a duplicate of the head model, with a negative scale applied. This allows you to focus on modeling one half of the subject while the reference reapplies your every move simultaneously.
Click the Edit button from the embedded scene below and modify the original mesh to see how the Reference follows your every move symmetrically.