I dont know if you can only open one instance of Effectrix.....just seams a logical answer if I've understood your question properly.

You'd use automation to tell it to start anyway would you not?
Have you tried automation on both Effectrixes to see if you really have one or two open?

OK you have 5 mixers, then it should be 'mixer 5', unless your sending it to channel 5 on A mixer!