Have back and forth syncing between main and sub site - USER SYNC.

I am working on getting the User Sync plugin to work with my sites Greenly.me and EasyMJdelivery.com. Greenly.me is the main site and EasyMJdelivery.com is a sub-site.

It is most important that any user from Greenly.me can be synced to EasyMJdelivery.com, but I would also like to make it so any user on EasyMJdelivery.com will be synced back to Greenly.

Is there anyway to make this happen?

This would mean that EasyMJdelivery.com would also somehow need to be seen as a main site to Greenly so user info created on EasyMJdelivery.com would sync back to Greenly.me and make any new users or edits to users mirrored.