Quick note about how to make this work. If you want to have two Unifi Security Gateways connect to a single controller at one location, you need to open up a couple of ports. Specifically, 8080 and 8443 need to be open to the controller. I strongly suggest you make sure you have a fixed IP at the remote side and you lock down the ACL (port forward) to only allow traffic to 8080 and 8443 from that remote public IP. Once you have that in place, you can have the remote USG be adopted by the controller's public IP. Be sure to add it to a different site.
After adoption is successful in the controller, turning on the site to site VPN is trivial. In Networks you create a new network. Select Site-to-site VPN from the "home" site network configuration. You should see the new remote site listed at the bottom. Simple as that.