If you're subscribing through Roku the devices must be linked to the same Roku account. Check the email address displayed under Settings->System->About on each device. If they don't match you'll have to factory reset one device to link it to the other account. There's no way to merge accounts. If they are already linked to the same account then you may want to contact support at https://support.roku.com/contactus so they can get to the bottom of it.
If you're not subscribing through Roku then it should be a simple matter of logging in with the credentials for that service on each device.
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