No, you’d still be able to connect a 2nd sub.  Note that, as it is currently, the two subs are playing the exact same audio.  Adding a 2nd sub give you move volume, not a 2nd channel of bass audio.

The naming convention for specifying audio channels gets a bit confusing as we’ve involved, in my opinion.  It originally was meant to describe the number of audio channels played, which happen to also be the number of separate speakers in the setup too.  Then of course, you have soundbars that placed 3 channels of audio in the same physical speaker. Then you add dsp/virtualizations that give the impression of a audio channel that isn’t actually there, such as the Sonos amp.  Now add in atmos, which really isn’t audio channels at all, but audio assigned to physical coordinates.

7.1.4 are the discrete channels, not the number of speakers.  Dual Subs both play the same channel.  Also, Atmos means the industry is moving away from discrete channels and into a positional object paradigm.  Pretty soon the concept of separate channels will disappear because the processor will position a sound in the room according to your particular setup, instead of simply pushing it to a channel which may or may not be optimally positioned in the room.