This is a common attitude from many, who think "gee my walls are rattling, why do I need ANOTHER subwoofer?" However, it's important to understand that the primary benefit of a second subwoofer isn't MORE bass, it's about BETTER bass.
The long wavelengths of the bass frequencies mean they are bouncing around the room and interacting, causing various modes (cancellations/nulls and peaks) because of the interaction of the waves. This is why, if you were to walk around the room, you'd hear the bass get stronger or softer at different places. The addition of more subwoofers can dramatically reduce the presence of these room modes, and the result is smoother, more CONSISTENT bass across the listening zone.
Especially with your 4311 having SubEQ HT for dual sub calibration, adding a second S12.3 would probably yield more consistent bass in the room, with less ringing, and thus you have have more articulate and accurate bass across the listening zone.
Some useful / educational links on the topic:
Optimizing dual subs perfectly requires measurement capabilities, but if you have a rectangular room the 1/4 + 3/4 width placements will almost certainly yield an improved result because of some fundamentals of the geometry of these modes: