How can I pair a second iOS device with my HomePod?

HomePod doesn't use Bluetooth pairing in the way you're familiar with. Setting up a HomePod makes an Apple Home if you don't already have one. All of your iOS devices signed into the same iCloud account should be able to access the Home. See Use the Home app on your iPhone, iPad, and iPod touch - Apple Support. See the Share Control section if you want to allow others control of your Home, or change the speakers settings as outlined in Change settings for HomePod - Apple Support if that is more appropriate.

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Go to the last section of this website Change settings for HomePod - Apple Support

What kind of iOS device and what are you trying to accomplish?