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Works here on OS 2...
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You can actually hold down the sleep/wake button on iPhone then press the home button to take a screenshot, but not vice versa (as holding down the home button activates Siri).
You can do the same thing with the Watch... hold the Friends button, then hit the Crown (doing it the other way works too, but you end up with a screenshot of Siri asking how she can help you).
 
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