The closer you can place your two stereo subwoofers to the main speaker , the higher in frequency you can do the crossover. 120 Hz is no problem if each stereo subwoofer is standing very close to the main speaker. 120 Hz is very troublesome If you use only one subwoofer . In a threeway floorstander with a 10 inch woofer placed 10 cm below a 4 inch midrange, theres no problem to crossover as high as 500 Hz in most cases.
Theres a connection between wavelenghts and the distance between the loudspeaker drivers. At higher frequencies , like a crossover at 3000 Hz, the distance between a midrange and a dometweeter must be very small to not be audible.
Now, one might argue that below the rooms transition area ( below 200 Hz ) you cant clearly hear the directions of the sound. The hard truth is that in reality, no single subwoofer have infinite steep crossoverfilter or have such low distortion in the driver themselves, that youre not gonna hear artefacts an octave or more above the crossover frequency . Those artefacts makes it easy for the listener to hear the location of the subwoofer. All real instruments also have overtones ( a bassguitar as high as 8 KHz ) and those overtones must arrive to the ear in exact time compared to the fundamental tones.
Subjective experiences:
My own experience with only one subwoofer , is if I follow the pitch of the tones played by a bass player , and I wont be able to hear the location of the single subwoofer, and no blurring is done to the bassplayers tones, I have to use a steep crossover ( 48dB/oct ) and not crossover higher than 55 Hz .*
This testing has been done with Genelec SAM subwoofers and before this with my own DIY dsp subs, with exactly the same result. With two stereo subwoofers, I can crossover much higher, 100 Hz is no problem if those stereo subwoofers are placed near each main speaker.
*It is absolutely true for me though, that with static sine tones ( without overtones ) I have much harder to hear the location of the single subwoofer, even at higher frequencies than 55 Hz. But this is not the way real music works.