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.
Yes. Some channels have a limitation as to the number of simultaneous streams per account, so you may not be able to watch it on both devices at the same time, but I can't say for that particular channel. I don't know how the subscription works for that channel, but if you're paying through Roku then as long as both devices are linked to the same Roku account (the email addresses shown under Settings->System->About should match) then the subscription will be valid on both devices. If you're paying BET+ directly then you just log in with whatever credentials you have with them and the Roku account doesn't matter.
If your BET+ subscription is direct to the channel provider, you will be able to access the subscription on all supported devices.
For more information about that channel's authentication and functionality, you'll want to contact BET+ support directly to inquire further. Many channels on Roku are developed and maintained by the channel provider themselves.