Two subs increases bass headroom by 6 dB, as well as significantly reduces peaks and nulls from room modes. You can’t EQ out a null, which can be 15 dB or more deep, and worse, the nulls are in different parts of the room, so even measuring them is tricky. Two (or more!) subs fill in for each other, since they are separated from each by many feet (preferably as far apart as possible).
A 6 dB increase in headroom is nothing to be sneezed at. 3 dB comes from having twice the power (two plate amps instead of one) and the in-phase in-room summing doubles speaker efficiency, so the net gain in headroom is 3+3 dB, a fourfold power gain. That’s equivalent to either getting 6 dB more efficient speakers (think horns) or 4X the power amp, assuming perfect loudspeakers with no power compression.
In reality, speakers with efficiencies in the 87 to 90 dB/meter range typically experience power compression with amplifiers more powerful than 100 watts, so a 500-watt Class D power amp is not necessarily the answer ... any loudspeaker will sound audibly "squashed" or distorted if too much goes in. Remember, a 92 dB/meter/watt speaker is only 1% efficient, with the other 99% of those expensive audiophile watts doing nothing more than heating the voice coils.