I am building a TWF Ninja for Pathfinder and I like the idea of using the double walking-stick katana, both for the option of using a Quarterstaff and the flavor of having hidden deadly weapons. The description for the weapons says that the blades inside the staff are closer to the wakizashi than to a katana. For the purposes of weapon focus, would weapon focus(wakizashi) work for when I use the blade or would I need to get weapon focus(double walking-stick katana)?
Usually, in this situation, the rules would say something like “Weapon Focus (wakizashi) also applies to this weapon” or something like that. In the absence of a statement like that, the default is no, Weapon Focus (wakizashi) does not apply, in which case you would need to take Weapon Focus (double walking-stick katana).
However, it probably should say something like that, in my opinion. Weapon Focus is a pretty weak feat; you really don’t need to be taking it repeatedly. It certainly sounds like Weapon Focus should be shared between the two.