Help getting started with Roku devices, including setup, connecting your device to your TV, linking a Roku device to your account at my.roku.com/link, adding channels, subscribing to services, and more.
Re: Additional charges for two Roku Streaming devices?
There are no charges for having Roku devices, period. You can add as many devices on your Roku account as you desire, and there is no cost for doing so. If someone tells you there's any sort of fee for registering your Roku devices, they are scammers and run away as fast as you can.
Now, what about content? For channels that charge a subscription (Netflix, Amazon Video, Hulu, etc.) you can install and use them on as many devices as you like. However, those companies might impose limits to the number of devices that can stream at the same time. I have Amazon Prime Video on over a dozen of different devices (Roku, Shield, Smart TV, Blu Ray player, iPad, iPhone), and I've never had an issue. Of course, I've most likely never had more than two devices streaming at the same time. Netflix has different tiers, with the lowest tier only permitting one SD stream, the next tier can have two HD streams, and their top tier allows 4 UHD streams.
One last mention. If all Roku devices are attached to the same Roku user account, they will all have the same channels (apps) installed. If you delete a channel from one it is removed from all. If you desire different devices to have different channels, they must be under separate Roku user accounts. That doesn't mean that one Roku device could log into Netflix with one account and another Roku use a different Netflix account. It's the channels themselves that are maintained within the Roku user account. Not any channel login credentials.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.