I think that started after Jim Lehrer's retirement, when Woodruff and the late Gwen Ifill succeeded him, not too long after the rebrand from just "The News Hour" to the "PBS NewsHour", still a few years before the 2015 set. I remember during Lehrer's last few years, even if the newscast had two anchors (Woodruff often co-anchored with Lehrer I believe), they would not be sitting at the same desk, one would be in one part of the set, while the other was on the opposite side, separated by a duratran, almost like to give the illusion that they were in seperate places.
That started once the new set debuted, with the social's rebranding to "PBS News" as well, even the website. Honestly, it's about time, especially in this digital age, the news operation should not be limited to just an hour, and it honestly makes things confusing in my opinion, imagine if the big three done that. "This is a CBS Evening News with Norah O'Donnell Special Report, I'm Norah O'Donnell in the nation's capital"