One simple* way to join two accounts: Pick one account to close. Cancel all subscriptions on that account. (Optionally wait for them all to expire – ie: they generally give you until the end of the billing cycle.) Then factory reset the roku device(s) on that (closing) account and put it/them on the account you are keeping. At this point they should automatically pick up channels and subscriptions from the account you kept.
*By “simple” I mean that it is a DYI solution that doesn’t require communications with support staff.
