It's worth noting that LW has a lower barrier to participation and a more community-centric focus, while OB has shifted to be explicitly Hanson's blog; these are clearly pursuing distinctly different goals. The simplest hypothesis is that Hanson and Yudkowsky disagreed on direction and decided to part ways.

In the About page on OB, Hanson says:

While we had a few dozen authors, most posts came from myself, Robin Hanson, and my fantastic co-blogger, Eliezer Yudkowsky. The topics eventually drifted more widely, and early in ‘09 Eliezer decided to move to a new sister blog, Less Wrong. I decided this was a good time to convert this to be my personal blog. My colleague Tyler Cowen had warned me from the start that blogs were best defined not by topic but by lead author personalities, and well, I’d learned he was right.

The phrasing here (especially the strongly complimentary reference to Eliezer) is a sort that, in my experience, often indicates a professional disagreement after which collaboration is ended, but with no real hard feelings over it and a desire to signal such to third parties. This suggests to me a lack of ulterior motives.

It does seem to be the case that the readers and commenters of LW and OB have some amount of non-overlap, but I suspect that is mostly incidental.