The blue post regarding PTR access is rather badly worded. What its stating is that you can't use the WoW account that was created for you to access the beta and you'll have to use another one for the normal PTR. Those that have been on the normal PTR realms before will already have one. Those who haven't can easily make one. When you both types, both accounts will show up in the PTR and beta clients but the PTR account will always go to the PTR realms and the beta one will always go to the beta realms irregardless of which client you use.
They should have just stated that those with beta access will need to make sure they have a normal PTR account set up as the beta one won't work for it.
The PTR is testing the 5.0.1 patch. Which means they'll be testing the pre-expansion event. You can't do that on beta.
Who says they can't enable that event on Beta, too? As it takes place in Kal/EK - the only significant changes to those continents were the Pandaren NPCs in the capitals (and maybe some out in the wild, though I only heard of that in pre-beta previews) though I even believe that unlike Cata, but like WotLK and BC, the world will already be in "Mists mode" with 5.0, we're just without access to expansion content until it is released.
People who are in BETA are testing Panderia Zones and Panderian Race
The PTR is for the EVENT PATCH that happens BEFORE Panderia goes live
2 Seperate testing options - dont get all excited too much as you wont be rolling level 1 Pandas in the PTR - you will get to see all the new spells and changes but not what they are seeing on the BETA servers.