Im using two identical 8 subwoofers with custom long-throw drivers and custom high-power class AB amps rated at 150 watts continuous and 400 watts + peak. I also own a 10 sub from a well known manufacturer that cannot compare with the 8 sub. The reason is that although the 8 sub is smaller, it uses a better amplifier and a higher quality driver which translates into more musical and a better performing subwoofer. Components used in this sub were selected for performance reasons without cutting costs to increase profit margins as manufacturers often do.
Recently, I auditioned a system that included four Peerless woofers (not active subwoofers) in a dipole setup with two per channel. The bass reproduction was awesome and the best I have ever heard. Using this a reference, the bass reproduction Im getting from my two 8 subwoofers is remarkable by comparison considering their size vs the fact that there were four 12 woofers total in the dipole setup driven by amps with 250 watts per channel into 8 ohms. So in answer to the above
Two smaller subwoofers can provide more stable/solid bass that would equal or surpass, etc. BUT, it depends on the quality of your subwoofers to begin with. In my case, two subwoofers have taken the sound to a higher level in every musically meaningful way: deeper, more robust bass that is more evenly distributed across the listening room; wider and deeper soundstage; improved clarity and definition of instruments and voices especially from the lower-mid through upper-mid ranges; overall the sound is more dynamic and alive.
Stereo or mono? Both of my subs are run in mono.
I agree with Bignerd, two smaller subs can do more work than one larger sub. Bass reproduction also seems to be more effortless with two. Two subs have no difficulty in
providing more than enough sound pressure to fill over 3500 cu. ft. of space in my listening room.