This question has been answered by treigh. See the first response.
You can technically set up multiple sites or subdomains on a single MODX install using contexts. ContextRouter might just be what you need and here's an article on the topic: http://www.cmstricks.com/blog/2013/05/10/contextrouter-review/
Here's another approach which uses the GatewayManager extra: https://docs.modx.com/revolution/2.x/administering-your-site/contexts/using-one-gateway-plugin-to-manage-multiple-domains
Hope this helps.