Open up the study you want to merge to another study.
Towards the bottom, click on the arrow that's labeled 'share & export' (next to the 'record' button).
From here you can either download the entire study or any specific chapter you have opened.
Next open up the PGN in 'notepad' or another comparable text reading software.
Copy and paste the PGN into the a new chapter in the second study.
Note: You may have to do this for each separate chapter instead of the whole study at one time.