ICloud - 2 iCloud emails - cannot access my 2 TB storage with 2nd email. Capped at 5GB

You can if you set up Family Sharing with the other account as a member. Read this article to learn how to share iCloud+ with your family --> Share iCloud+ with your family - Apple Support

To learn how to set up Family Sharing, read this support article --> Set up Family Sharing - Apple Support

That's what you would have to do, yes. Purcahses made with one account can only be accessed by another account through Family Sharing.