Extension won't improve using two Klipsch subs. The R-112 will have extension down to the mid to upper 20Hz range. Two subs will deliver more output, however, and give you more even room coverage when properly placed compared to a single sub.
You might keep an eye out for the Monolith THX Select to go on sale again. It's the best sub out of this group, followed by the PB-1000. Both of those are 20Hz subs and the others are in the upper 20Hz to 30Hz +/-3dB extension range. The Monolith 10 was just recently available for $375, which was one of the best deals on quality subs so far this year. I had the Jamo J-112, which is basically a variation of the Klipsch sub-112 using the same amp. Supposedly Klipsch has finally addressed the issue of amp failures that have plagued many of their models, including the R-112. The R-112 will not only fall short of the Monolith and SVS in terms of extension, but it's also not nearly as accurate. The Speedwoofer has free returns if you want to try it, but it only has extension down to around 30Hz +/- 3dB. It will have a more accurate sound than the Klipsch, however.
Subs>RBH I-12e, I-12. HSU VTF-15H mk2. Monolith THX 12(x2), THX 10(x2). XTZ 1X12. SVS PB-1000(x2). Speakers>JBL Studio 270/235c/230/225c. Sony CS3/CS8/CS5. QA 3020i/3090Ci, 2020i/2000c. Monolith Cinema 5bs. Polk RTiA1/CSiA4. Other Audio>Sony MDR-Z7m2,V6, XB900n,700,XB32,XB31. Sennheiser HD58X,HD4.50,PC37X. HiFiMAN HE4XX. Philips SHP9500. SIVGA SV004. SA HDTwo. HyperX CloudPS4. TB Elite AtlasAero. LG FH6,RK8,RK7,PK5. HK Onyx Studio4(x2).