Yes, a stereo source plugged into either of a stereo pair of Fives will play in stereo to both speakers. A different source could be plugged into the other Five too, and also play in stereo when selected.
If you feed a stereo jack into one one of the speakers (preferably the left but either is OK) then a stereo pair will play in stereo as for any other stereo source. Indeed, you could feed different sources into each of the lines-in and play either of them on any “room” in your Sonos system.
If you have other rooms, one source could play to that room too, whilst the other is in use by the stereo pair.
@John B beat me to this added note 
@nik9669a beat me to it. Must learn to type faster. Or think faster.
@nik9669a beat me to it. Must learn to type faster. Or think faster.
I didn’t emphasise a stereo cable with stereo jacks is needed, though. 
...(preferably the left but either is OK) ...
Many thanks for ultra fast response. Highly appreciated. Quick follow-up question: Any reason why the left is preferred?
@nik9669a, @John B : And here is a follow-up question on stereo: In https://support.sonos.com/en-us/article/selecting-a-location-for-your-sonos-speakers it is stated:
“A horizontal speaker will output stereo audio over a wide soundstage while a vertical speaker will output mono audio. When placing a single speaker, we recommend using a horizontal orientation.”
I would assume that when there is a pair of two in stereo mode this will not matter as inidcated in the 2nd sentence.
No: either orientation for a stereo pair, but vertical for a pair is more “conventional”.
A Line-In is simply a network resource that can be selected for play on any combination of speakers. Note that a Line-In will not automatically play on its speaker — because Line-In is a generalized network resource that must be selected.
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For the FIVE:
A single horizontal FIVE will play stereo.
A single vertical FIVE will play mono.
A horizontal pair of FIVE’s will play stereo with the level on the inside facing tweeters adjusted to improve stereo imaging.
A vertical pair of FIVE’s will play stereo.
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For any pair of speakers the left speaker becomes the pair’s “Coordinator”. All network traffic for the pair flows through the Coordinator.
