A single server can serve as a witness for multiple DAGs. However, each DAG requires its own witness directory.

This recently trip us up when bringing up new 2016 servers in our existing 2013 environment. We and used the same witness directory for both DAGS (e.g. \\MGMT500\C$\DAGFileShareWitnesses) under the assumption that the witness directories would be different.  While it does create a folder with a different GUID for each DAG. it treats the full path as owned by either DAG. So if any issues occur and the DAG decides to remove the fileshare witness directory it will remove the whole path. So if you are going to use the same server for your fileshare witness make sure to specify a different path for each.