On the 15401-2-0-1-0, half the fuses are tied to one common +12V rail and the other half to the second common +12V rail. One rail could be always-on and the other could be only on when the ignition switch is on, or they could both be always on, or both always switchede; it all depends on what you connect the rail to in terms of power.
The advantage of the dual bus is you'll save a little money and a lot of time, trouble and mess because it's all pre-wired internally. The disadvantage is that you have a max of 100A per bus. You need to be careful not to exceed that rating, and should put an appropriate fuse in-line with the connection between the PDU and power source to avoid melting/shorts/FIRE.
I'm a JKO Reject