Here's the thing about "stereo." Two-channel stereo is artificial to begin with - two speakers trying to mimic a much more complex live musical environment. What Apple is doing here is not a mono speaker (which would take the sum of the left and right channel signals, resulting in a flat non-directional experience) - it's a multi-speaker array (single woofer, 7 separately-driven directional horn tweeters) that, with the help of tons of computer processing and direct/reflected sound from those tweeters, creates a complex sound field from a single box (well, cylinder). This takes both channels of a stereo signal and uses all of it. The addition of a second unit will allow for an even more complex sound field.
The principles behind this have been around for a very, very long time. We've seen many implementations come and (sometimes) go. Quadraphonic, 5-channel and 7-channel surround, sound bars, Bose Direct/Reflecting speakers, self-tuning sound systems... All these were done with cheap, passive speaker/amp arrays, with relatively little or no active signal processing. There's also nothing new about beam-forming tweeter arrays. A dual-HomePod array should produce the equivalent of super-stereo, accomplishing with two units what others may hope to accomplish with 5-speaker systems.
The main difference is that Apple is able to take these established principles, add their proprietary R&D, apply a ton of active digital signal processing, and execute it all in a mass-market product that sells for $350. What they're applying here has been learned giving iPhones, iPads, AirPods, and iMacs remarkably good sound (for what they are), and no doubt, what they've been learning for HomePod will also flow to Apple's/Beats other products.
Oh, and P.S., yes, HomePod will be controllable by multiple people (which is to say, more than one person can issue voice commands).