Currently, you can't share a File Manager between two sites, but perhaps this will work instead:
1. Select all of the files or folders in the File Manager that you would like to upload to the other site. You can select more than one by holding down your Shift or CTRL key if you want to select all or only some files.
2. Right-click on the selection and choose Download as zip.
3. Download the zip file to your computer.
4. Open your other site and click Files.
5. Upload the zip file and right-click - choose Unzip.
All of your files and folders will be added to your File Manager very neatly!
View this tutorial: Using the File Manager.