Try a different approach. Instead of using plugins, I suggest modifying wordpress a little as described in the following answer.
https://stackoverflow.com/a/39195424/3157038
So in your case you should setup the wordpress installations like this:
- mysite.com
- root: */domains/mysite.com/public_html
- db: user_mysite
- table prefix: root_
- mysite.com/itsupport
- root: */domains/mysite.com/public_html/itsupport
- db: user_mysite
- table prefix: itsupport_
than in addition to the configuration given in the answer which i linked to, add the following to the wp-config files of both the wordpress installations:
define( 'CUSTOM_USER_TABLE', 'mysite_users );
define( 'CUSTOM_USER_META_TABLE', 'mysite_usermeta' );