We had a slightly different scenario, so I wanted to chime in as I could see it being helpful for others in the future. We had two MS225 that had previously been physically stacked using a single stacking cable. We ended up with an extra stacking cable from the home office, and wanted to add that to the pair. It wasn't clear to me whether we needed to power down the stack, or if it would disrupt the stack if we added the second cable while everything was up and running. I waited for an early morning and tried adding the cable with everything live. It took about 3-5 minutes for the dashboard to show both stacking ports on both switches as active, but otherwise, it was super smooth and non-disruptive.
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