At the moment, I have my 608 healing cloak for my holy pally. However, I'm going to be going prot in 5.4 as a main tank. I already know that you can get a second 600 cloak in the PTR, but I'm not sure how it works in terms of the legendary cloak. As in, if you make one cloak legendary can you buy the rest as legendaries? I could make my healing cloak legendary and then buy the cloak for prot. Or is it only one cloak that can be legendary? In which case, I'd buy my prot cloak now and make it legendary, keeping my 608 healing cloak.
You purchase the cloaks after you complete the quest chain as Mokoshne said, I think it's like 500g or something like that, I could be wrong but I haven't tried to upgrade another cloak as I haven't got the legendary on the PTR because I'm just waiting for 5.4 to hit live.