It would have been simpler and easier to have just linked the second Roku to the same Roku account as the first one. They'd both have all the same channels, but that's water under the bridge now.
Once set up a Roku remains connected to the same Roku account (identified by email address) as it is linked to. You can't just switch back and forth between them.
To change Roku accounts you have to do a Factory Reset and set the Roku up anew, this time linking to the other Roku account (email address). Any premium subscriptions you may have through Roku on the original account will not transfer to the second account.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X