After 8 hours of running streams, it does not appear possible to get more than one of each drop and it's simply a single one of each for the entire campaign. I know exactly where you're coming from because I also read the italicized end note as literally saying that you can one of each drop for every 60 minutes streamed. I think what it's supposed to mean is that (if you do not have both drops already) every 60 minutes of view time you will get a 50% chance for the other item and you may not necessarily get both drops in two hours because the 50% roll may land on the item you already received. So just keep watching until you eventually get both items.
If that's the case I think it would be better put as, "Watch any Warframe Partner stream for 60 minutes for a 50% chance to receive either the Cherry Tree Glyph or a built Forma. [Every 60 minutes of additional view time will give you another chance for the other item to drop.] Limit one of each."