+1It does indeed; if the depth and width are the same it becomes a bigger problem. However, modes can still (largely) be overcome even in that situation. For example, corner-loading one sub and then having another mid-wall on the opposite side of the room might do the trick in some cases. Placement is key though. It's why I laugh when people say dual subs are always better than a single unit. That's not true if you can't place them properly.
Dual sub only helps if one has the freedom of space to place the subs at the right positions. Else, it only make thing worse.