Agelong Tree allows you to join two data files, see the menu File – Append data from:

If the file to be added was previously a copy of the current file and there are changes in the shared data in the added file, these changes will not be taken into account when adding data, only new data will be added.

After adding data from another file, you may need to delete duplicate persons and merge people using events, you can read about itin Questions and answers.