ArcGIS Pro Unmerge?

[Rey_A_Santiago]

How do you undo a merge in ArcGIS Pro?

[DanPatterson_Retired]

Too many 'merge' options.

If you mean

Merge—Data Management toolbox | Documentation 

and specified an "add_source" or some other unique field is in each table,

Then 

Split By Attributes—Help | Documentation 

might be able to do it if there is that field information which can be used to differentiate the original files (assuming you have no backups, I am guessing)

[Rey_A_Santiago]

In the original layer all the the lines in a parcel were merged. I have another layer that contains the parcel in its original form. Can I merge or append this second layer into the original and save in the original? Append attribute tables?

[DanPatterson_Retired]

Are you using the tool that I posted? and I thought you wanted to undo the merge? Can you clarify what your inputs are and what you want as an output?