A Discourse instance can function as the SSO provider for another Discourse instance. I am assuming that the setup you are considering is having WordPress as the SSO provider for Discourse instance 1. Discourse instance 1 would be the SSO provider for Discourse instance 2. I think this is possible, but have never configured a Discourse site to be both an SSO client and an SSO provider.
With the setup I outlined above, all users on Discourse instance 1 would have access to Discourse instance 2. I don’t thing that is what you are wanting.
Another possible approach to the problem would be to use one instance of Discourse and use category group permissions to limit what parts of the forum users can access. You can pass Discourse groups in the SSO payload. It should be possible on your WordPress site to determine which sites a user has access to. You could create a Discourse group for each WordPress site and then use the wpdc_sso_params filter to add an add_groups parameter to the SSO payload.