I'm pretty sure the riocar Disk Upgrade Guide lists precise step-by-step details to do what you want. I might have been missing something, though, let me know what it is about what you're doing that the Disk Upgrade Guide can't help you with?

(Note: Your case of wanting to copy the "stuff" from two smaller drives onto a single larger drive is covered in there. It's one of the bullet points in the section about copying files.)