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Have a look at the image gallery, the 3rd image shows the offset left and right variants used in the middle of the JPC plate carrier (which has 6 columns of PALS webbing in total). The offset left and right take up approximately 4 columns of PALS -- this mag holder design is quite chunky so unfortunately they take up a lot of space.
Generally speaking, if you want to maximise the available space I'd suggest using the offset left and right mag holders together. If you used 2 of the "regular" centered mag holders side-by-side, they would actually take up almost 5 columns of PALS webbing, so... it's all situational. Does that make sense?
Also, running both Talons and Katanas interchangeably in the same mag holder is possible, but note that the ideal shock cord tension is different for both, as Talons are significantly larger -- i.e. if you tension the cord for Talons, it'll be a very loose fit for Katanas, and vice versa. Just something to be aware of.
Let me know if that doesn't answer your questions and I'll do my best to provide more info.