Eccentrics for Walshert's Valve gear

I am trying to understand eccentrics and how they should be aligned on Walshert's Valve gear.

A good book for this appears to be
http://www.google.com/books?id=TKV-AAAAMAAJ&printsec=

As I understand it the proper way to refer to this alignment is outside and inside admission. What I do not understand is when one would be used over the other and on an articulated is it would be proper to have both on the same locomotive.

In the following photo which engine eccentric would be inside and which outside?


Notice on the photo of a model of a model NG compound mallet that the front engine and rear engine have the eccentric aligned differently. While this does not appear to effect the operation of the model in any way what would be the advantage or disadvantage of doing this in the prototype?

I still have a lot to learn about the alignment of steam locomotives and any help would be appreciated.

Thanks
Stan