I decided to go with lists.tssi.com for the gateway site and x.tssi.com and y.tssi.com for the separate communities as opposed to x.lists.tssi.com and y.lists.tssi.com. Couldn’t get the latter to work, possibly an ngingx setup issue, it kept rerouting requests for x.lists.tssi.com or y.lists.tssi.com to lists.tssi.com rather than to the Discourse server.
But I can get the other to work, and a working solution is better than a non-working one. 