You only need the Internet for initial set up. Then it can be disconnected. Otherwise there is no way of setting them up and you must pose the S1 app version

How would they be connected if there was no internet after initial setup?

If you bond them as a stereo pair, and configure the Line-In Autoplay to target the pair, they should work in the absence of a network. (The two units should connect to one another using an ad hoc SonosNet mesh and AutoIP addressing.)

Try it and see. Configure using the S1 controller on a normal network, then turn off the router and power-cycle the units. (I haven’t tested this in many a year.)

So that’s the only way to play them in stereo? What about using a headphone splitter to play them as a pair?

A headphone splitter would send L+R to both units. If each was configured with Line-In Autoplay it would play individually, but the two wouldn’t function as a stereo pair.

If you were to butcher the splitter so that its L+R were divided/combined so as to send L+L to one speaker and R+R to the other then it would achieve much the same result as my earlier post, though the stereo sync could be slightly off at times.