That's why it's a golden rule: as you say, one sub is never perfect. As you say, it's a compromise. It doesn't have to be.
To unpack that a bit, to see why it's true, consider that real in-room bass is nowhere near as 'non-directional' as it's sometimes suggested. It's always possible to locate one LF source in a 'lopsided' presentation.
That's partly because LF emanating from a single point source instantly generates stubbornly fixed standing waves, triggers nodes and creates peaks and troughs that permeate your room like a photograph of ripples in a pond.
In fact, that's not a bad metaphor . . . drop a single stone into water and a regular pattern of rings form. Drop two stones and the ripples interact chaotically, partially canceling. Situating two LF generators in and out of nodes - even without DSP and EQ - just by a bit of trial and error - instantly improves the evenness of LF response. It's the right way to do it. It's why all high end cinema systems are .2. It's why active speakers generate such clean, natural LF. It's related to the excellence of bass from open baffles.
Trying to get good LF from a single sub is a fool's errand relative to doing it right in the first place - which just works. EQ and DSP are like putting plasters on a broken leg. Why not fix the leg?