How to add more than one voice on hey siri

At any given time only one voice can be used.

Change the voice for Siri

Go to Settings > Siri & Search. Tap Siri Voice, then choose a different variety or voice.

Change Siri settings on iPhone - Apple Support (IN)

sebert144 Said:

"How to add more than one voice on hey siri"

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You Can't.

So, Recommend this to Apple:

Do your Part: Provide Apple with feedback on this, letting them that you want this to be an option.  Apple may not get back to you directly, but the more feedback they receive on this, the more they will know what their customers prefer, and what features to include in versions to come.

1. Go Here: Feedback - iPhone - Apple
2. Select: "Bug Report" for the "Feedback Type"
3. Comment: that this all began after installing an update. Include the URL of this post
4. Proceed from there as necessary