Yes. the Sonos Five speakers do work horizontally, as well as vertically as a stereo pair. The support documentation says this:

Speaker orientation guidelines

You can place a Sonos Five, Play:5 (gen2) or Play:3 in either a horizontal or vertical orientation.

A horizontal speaker will output stereo audio over a wide soundstage while a vertical speaker will output mono audio/single audio channel. When placing a single speaker, Sonos recommend using a horizontal orientation. 

When either horizontal or vertical, make sure all four rubber feet are sitting completely on the flat surface.

I will just add to the above with this slight clarification…

A single vertical speaker will not play in stereo (it’s mono audio only), whereas a single horizontal will have a left/right channel. Two speakers paired in stereo will play in stereo regardless of orientation.

If using the Fives as a stereo pair, then orient vertically with tweeters on the inside for a sharper stereo image, or put the tweeters on the outside for a slightly wider, but possibly more diffuse soundstage.

Hope that helps.