Two subs, yes or no?

If you are using two small 'monitors' having two subs will allow you place the subs close enuf to the monitors so that stereo signals coming from the sub (above the nominal cross over point) will integrate with the same sound from the monitor and it will be more seamless.

If you are using the subs to reinforce or supplement low frequencies of floor standers using two subs will allow you more freedom in placement in the room to avoid large nodes. You run the mains full range and then adjust the output level and cross over frequency, phase, etc, to match depending on where you put them in the room. You can usually do a pretty good job with just one, but often two just works better.

FWIW.

If a sub is operating at its intended range, you should really only need two when the output from one is not enough. Subs should be crossed over as low as possible, and I believe anything below around 80 Hz is omnidirectional. Meaning, humans can't tell where the sound is coming from. (Aside from placement issues)