Main benefits of two subs is smoother response across the room plus of course twice the drive for the lowest octaves, which benefit the most from additional power and drive.
Crossing over to subs for lowest octaves also helps take load off main amp and speakers and generally results in better performance there than otherwise as well.
Stereo imaging gradually becomes less of a phenomena at lower frequencies, but I would expect some benefit there as well, especially if subs are crossed over in a manner that offloads more rather than less of the bass. So its less of a benefit at lower frequencies, but not completely a non issue, yet one that alone probably would not carry as much weight but can only be better and not worse with two subs rather than one.
So you can see that using one big sub rather than two smaller ones is a decent option still that works perfectly well for many even though there may be tangible benefit still in many cases with multiple subs.
In the case with seemingly fairly full range speakers already like the OPs appear to be, probably only larger and more powerful subs capable of substantial output 30 hz and below will be off benefit.
Many smaller subs are designed to supplement smaller monitor type speakers with limited bass extension already. These will add little if any value if used with good quality more full range speakers already. So larger good quality subs that use larger drivers (12 inch or larger most likely) are probably needed for true benefit and good ones will also tend to cost more accordingly. Check out frequency response specifications or even better any published measurements if available to help confirm that a particular sub has the low end extension and response needed to add value to an already mostly full range system.
Crossing over to subs for lowest octaves also helps take load off main amp and speakers and generally results in better performance there than otherwise as well.
Stereo imaging gradually becomes less of a phenomena at lower frequencies, but I would expect some benefit there as well, especially if subs are crossed over in a manner that offloads more rather than less of the bass. So its less of a benefit at lower frequencies, but not completely a non issue, yet one that alone probably would not carry as much weight but can only be better and not worse with two subs rather than one.
So you can see that using one big sub rather than two smaller ones is a decent option still that works perfectly well for many even though there may be tangible benefit still in many cases with multiple subs.
In the case with seemingly fairly full range speakers already like the OPs appear to be, probably only larger and more powerful subs capable of substantial output 30 hz and below will be off benefit.
Many smaller subs are designed to supplement smaller monitor type speakers with limited bass extension already. These will add little if any value if used with good quality more full range speakers already. So larger good quality subs that use larger drivers (12 inch or larger most likely) are probably needed for true benefit and good ones will also tend to cost more accordingly. Check out frequency response specifications or even better any published measurements if available to help confirm that a particular sub has the low end extension and response needed to add value to an already mostly full range system.