But TTN is not a one-author series.
It hasn't been to date, but if the current editorial regime wanted it to be, there's no reason it couldn't become one -- just as there's no reason that, say, Voyager
couldn't become a multi-author series if the editor wanted. Indeed, if Marco had stayed at Pocket, VGR would've become multi-author after Kirsten's duology; he'd indicated to me that he was thinking about recruiting me to do the third book. But once he was laid off, plans changed, and the subsequent editors chose to stick with Kirsten. (Which turned out to be the right choice, since otherwise we would've been cheated out of Children of the Storm
So it's not like there's some rule requiring certain series to be single-author and others to be multi-author. I have no idea what the future plans for Titan
are, but it's all up to editorial discretion.