I assume DC hope that Nightwing readers will pick up the Titans book after one arc of showing the team operating in Bludhaven.
Currently Nightwing is one of DC's top selling books, while books like Titans United (that notably is out of continuity, but still) hardly sell. And it's not just this book. Almost all of DC's new books are going to be minis. Green arrow is getting a mini, Jon Kent is getting a mini, even bat-brands that should be strong sellers like Robin and Batgirls are struggling with sales and aren't even in the top 100.
So I assume that DC are trying to play it safe. They attach the Titans book to one of their top sellers and hope that it will lead Titans fans to read Nightwing, and later that Nightwing fans to pick up the new Titans book.
And from the solicitations it seems that Nightwing 100 is continuing just after Dark crisis, so if we consider the Nightwing book as a prelude to the Titans book, there's basically no delay in them stepping up.