Be aware, based on reviews/accounts from the media preview Diagon Alley is very dense with stuff to do/look at. London, Nocturne, the rides, the shows (including knight bus, Olivanders, and the other two), interactive elements in all of them) As potter fans you could easily spend half a day just there, and this is not not including the original world (Hogsmeade). The layout of Diagon is like a big figure 8, where two of the shows are is in Carkitt Market, which is a outdoor covered area that JK ROwling created to flrsh out Diagon Alley (I think it may even have A/C).
I have a feeling a lot of people trying to stuff both parks in one day are going to easily run out of time, especially the first year of Diagon and first time visitors.