What frequency is this void? If it’s around the crossover frequency your problem is the phase matching is not right.
If your room is rectangular, try the Room EQ Wizard’s room simulator, see if you have similar results, and then you can tell how much a second sub would help.
If your room is rectangular, try the Room EQ Wizard’s room simulator, see if you have similar results, and then you can tell how much a second sub would help.