If a subject is being seen at site A and would like to continue care at site B. You can create a site called “A-B” that would allow both sites access to the participants for the subject. 

1. Navigate to the settings 
2. Click on study 
3. Click Manage sites
4. Add site “A-B”

After which you can assign the subject to the new “A-B” site. 

Please note users from each site will need the necessary User Rights to be able to view the Participants in the new site.