I think, generally, the idea was for the hosts to cover major stories like what Mel, Kochie, Karl and Lisa have done previously to correlate the big hosts covering their big stories. More recently, as mentioned, both shows have changed to the news presenters covering the big news for two different reasons. IMO, Nat is removed from the studio because I believe she is Pell's next 'victim' whereas Sylvia is appointed this 'big' news events (I believe reporting is her strength) to basically make her more popular.
BTW, I swear that Today has 'special coverage' every day for trivial stories. It is getting worse than the 'breaking news' coverage.