I just installed workflow management.

All my sites are located under http://spse/sites

I tried to configure it using this command: Register-SPWorkflowService -SPSite "http://spse/sites" -WorkflowHostUri "http://spse:12291" AllowOAuthHttp

This failed. Why?

So I use the same command pointing to one of the site: Register-SPWorkflowService -SPSite "http://spse/sites/demo" -WorkflowHostUri "http://spse:12291" AllowOAuthHttp

This succeeded.

I tried to register another site: Register-SPWorkflowService -SPSite "http://spse/sites/demo2" -WorkflowHostUri "http://spse:12291" AllowOAuthHttp

That failed. I am getting the following error: Register-SPWorkflowService : Failed to register because the farm or partition is already registered with a workflow service

So I have two issues to resolve:

How to unregister http://spse/sites/demo

How to register all the sites I have under http://spse/sites

• Moved by Hemendra AgrawalMVP 5 hours 0 minutes ago SP 2013

Given both Demo and Demo2 site collections are in the same web applications you do not need to register your workflow service again. 

Basically the idea is you register the workflow service once by providing one site collection as an argument and that service will be available for all site collections in that particular web application.

If you have other web applications and they are using default service application association group then you will have your workflow service available for those web application as well.

So to answer you question.

1) You do not need to unregister your workflow service.

2) As mentioned above you do no need to register workflow service again

a) They are under the same web application

b) Using the default service application association even on the different web application.