Hey @Zapatilla,
both codes will not stack together for your brother, but what he could do instead is giving away the premier code that he would receive with the premiere edition:
"If you upgrade from Buddy and purchase a Premiere edition, you will receive a new Premiere code via email which you can give to a friend. The new Premiere code cannot be stacked upon your current Buddy Pass." Source: This article
Hey @Zapatilla,
both codes will not stack together for your brother, but what he could do instead is giving away the premier code that he would receive with the premiere edition:
"If you upgrade from Buddy and purchase a Premiere edition, you will receive a new Premiere code via email which you can give to a friend. The new Premiere code cannot be stacked upon your current Buddy Pass." Source: This article