First, create the folder/directory or the subdomain you want to use for your WordPress uploads, make sure it has the correct permissions and is writable.
In your wp-config.php file which is in the root of your WordPress installations, you can add the code below. Do this on both your WordPress installations and modify the location to that of yours.
Make sure you place that line somewhere just above this line:
This will now set the custom location for your uploads to be the same location on both installations.
However, when you upload files to the server using WordPress uploader, it makes a record of it in the database. The media library references what's in the database, not what's in your file system.
So you will now find that images uploaded via the media library on the other WordPress installation are not visible in the other WordPress media library even though you will find the images exist if your browse to your uploads location on the server via FTP.
You would have to write something from scratch that scans through all the images within the new uploads sub directory and adds references to these into the database on the site that was not used to upload them.
I have also checked and found this for you. I have not used it but looks promising. Add From Server plugin