it all depends in the source output. Some sources, rare though will have separate subwoofer outputs for left and right channels. But the most common is the monoaural single subwoofer output. Even this can be split and fed separately to left and right channels.
Yes, two dedicated subs will technically be superior to a single one, but that much more difficult to place and integrate. A single subwoofer is the most practical, in terms of integration, cost and most importantly space.
In my setup I have used a single sub kept at the center. I don't have a dedicated sub out on the source, so xo is set on the active subwoofer and I have done the integration with my ears with respect to matching sound level, crossover and phase with my bookshelf speakers.
Similarly in my car audio, due to constrain of space and power, I have used two 7 inch midbass in an isobaric configuration as a subwoofer. It is in the boot and powered by a 50 watt RMS TPA3116D2 board. The integration is done on my active Eclipse head unit
Kannan, have you modified your Infinity Book shelves to match the XO which you set on the Sub? Thanks.