It depends on the room and where you're seated. There are no hard and fast rules when it comes to sub placement. For example, let's say your room and your seating position is fairly symmetric and you place a sub in the front corner and another in the other front corner, then you would likely get a very similar response as just one sub in the front though you may get more output overall. Placing them equidistant like that is a safe bet in terms of keeping them in phase, however, it could actually severely restrict you in options that can actually smooth out the response more effectively. If you do try out different placements, such as one in the front and back, then one of the things that can really come in handy is a receiver with dedicated bass management for both sub outs. So that means you can adjust the distance for each sub individually. That will open up new options and allow you to keep the subs more in phase with each other.