I guess the safest, fastest, most gradual way to do it would go like this:
Go ahead and get your brand site up and running as a stand alone site on a different host with different registration info. Work on creating content and a link profile as separate from the others as you can. Set up a google+ profile verified for that domain and begin ramping up social media efforts for it.
Then, start pumping good content onto your emd's that gradually link out to numerous other quality resources, including, once in a while, the quality content you're putting on your brand domain.
With this method, you'll hope to get traction for your brand domain before your emd's lose value (if they do). But if the emd's do lose value before they can pass full 301 juice to your brand domain, your brand site is already building a solid base for itself.
On the other hand, the fastest way to deal with it would be to create your brand site and then to go ahead and 301 each page from your emd's to the appropriate page on your brand site and get the benefit of those two sites 301ing to the brand for as long as that benefit lasted.
With this method, you'd certainly expect to get traction for your brand domain before you emd's lose value (if, in fact, they did), but you'd lose out on the benefit of having those two emd's in the serps at the same time--for the period of time that they didn't actually loose value (if they did--but you'd never be able to actually determine that.)
As the executive decision maker, you get to make that call.
Does that make sense?