Your pre-existing speaker is probably a stereo speaker.  Instead of just playing mono, or the L/R side of stereo audio, it will play both the left and right side of stereo. Two wires for the left channel and 2 wires for the right channel.

So I would either get a 2nd replacement speaker, and place the 2 new speakers in new positions for a stereo image...or return the replacement speaker and get a stereo speaker.  If you google “outdoor speaker stereo input”, you’ll see lots of options.  I suppose you could put the amp in ‘mono mode’ and it likely would work, but would not sound as good as the other two options.

Sonos does sell outdoor passive (requires amp) speakers, but not with stereo input.  You definitely can use a stereo input passive speaker with the Sonos amp. And, yet another option would be to use 1 or 2 of Sonos portable Roam or Move.