More than one path alias per node I have a ticketing system where the path aliases are set like ticket/[node ID], e.g. http://example.com/ticket/1234.

I would like to create multiple path aliases for the same node, and be able to access it also as discussion/[node ID], e.g. http://example.com/discussion/1234.

I will be using different node templates basing on the path.

Is there any way to set multiple path aliases for the same node?
You can set multiple path aliases for one node, this is no problem. If you can't do it with Pathauto, then it's possible with custom code when saving the node.

But in your case it might be more efficient to set up different routes for /ticket/{node} and /discussion/{node}. You can either set a fixed route parameter or use a custom node controller to set a marker, that you can use in theme name suggestion to choose the custom template.

Another solution would be to set up a View with a contextual filter for the node and two displays with different paths. Choose for each a different view mode to get different templates. Technically it's almost the same, but you need no custom module, because Views sets up the routes for you.