Thanks for reaching out to the Community and welcome.
It's possible that both Alexa devices have the same Spotify account linked to it, or both speakers are paired. For this reason, we recommend that your daughters or you make sure the speakers aren't paired.
In order to unpair them, head to the Amazon Alexa app and tap on Devices > Select the group of linked speakers > tap on Unpair.
Lastly, unlink Spotify from Alexa and link again each Spotify account to the desired speaker (the Spotify credentials will be requested when linking Alexa to Spotify). There's more info here.
Let us know if you need more help.
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"Music touches us emotionally, where words alone can't." -Henry Thoreau