SSIS Indirect Configurations

With indirect configurations SSIS enables you to store the path to the dtsConfig file in an environment variable.  At runtime the package looks up the location to the config file, load the config file at that location, and does whatever it needs to do with the information.

If your SSIS package gets executed from a SQL Job, it is important to restart the SQL Agent after creating the environment variable - the SQL Agent caches the environment variables and won't pick up new ones unless restarted.

Multiple SSIS instances on the same machine

You can only have one instance of SSIS (SQL Server Integration Services) installed on a machine at a time.

But you can have multiple SSIS database instances on one machine at the same time. See this post on how to do that.