You should deactivate the setting "Common resources" on the Importer tab. Also make sure that no Worker is requested for Processing tasks.
From Plant 13 (maybe 12.2 also, not sure), you can also de-activate the Processing-Importer independently from the Setup-Importer. But in early versions there was a bug and this setting could not be saved, it would always go back to both importers active. So make sure that you are using the latest patch!
I am using plantsim 11 and i dont have this problem. I can choose seperate operator for setup and seperate for processing.
Open importer tab:
uncheck common resources--> click setting up radio button-->open service for setting-up-->put service "Setup" & amount 1--> apply-->click processing radio button-->remove all service/delete table content-->apply-->ok
CLICKING APPLY AFTER SETTING UP IS IMPORTANT.
put a broker and worker pool with operator service as "Setup" also in the model. Remember to put broker seperately in both the services. Check the model attached.
I just wanted to let you/the OP know about a new option, in case you/the OP are trying to use it and the bug of the first versions is causing the issue (since the OP didn't state which version he/she uses or what exactly he/she did, I thought that bug could have been the issue, because of the symptom "as soon as I close the single-proc window it comes back to processing").
Here's what I was referring to: you have individual checkboxes for both importers, to (de)activate them individually: