If I were to re-arrange my tank to house 2 emps would I also have to change the substrate? right now there are a few places were my current scorpion has left his wastethey should be roughly the same size. you will have to watch to make sure the current resident of the tank doesn't take offe3nse to the new addition, unless you re-arrange the tank so it appears as new territory. (anyways that's what works best for me). there is a slight chance of aggression between the two, but usually it is just territorial displays and establishing a pecking order.