I think the problem with the last series was that the housemates got bored, as they didn't really have anything to do. I think the two house concept will liven things up if it does go ahead, as everyone will be wondering who's "next door".
And I guess that could be the reason for the extra week.
It must be to allow for a sift of housemates - though 2 houses is a big expense for 1-2 weeks which suggests a second house might have another role?
It has other advantages - if the audience picks the housemates the producers make more money on votes and they escape the blame if the housemates picked prove dire. It also means C4 would have another 2 weeks of programmes to fill their schedule which is looking thin since Brookside, RISE and Norton have all been taken out of it and Friends is soon to join them.