Have you ever tried to 'Copy and Paste' a REVIT legend from a 'Sheet View' in Project 'A' to a 'Sheet View' in Project 'B' ?
It cannot be done or at least not from 'Sheet View' to 'Sheet View'. So how can you copy a legend from one project to the next ??
The answer is to 'Copy and Paste' all the elements from Legend View to Legend View. Not from Sheet to Sheet.
Once you have pasted the elemets into the Legend View of Project 'B' then you place that legend on a sheet.