A Challenge: Designing A new modular layout section

[torikoos]

As for the "main" line if these are to be paired modules the only place you would really need the 9" centers would be at the extreme ends of the module set.

I have to ask the same question, which begs the next question - - - would your "two module SET" ever have to be separated, and placed in different locations ?

no, in fact, a technically correct term would be to call the two sections 'boards', that together make up a 'module'.
So my module, consists of two boards, that are not planned to be separated. However, as a pair, they could end up anywhere in a modular set up.

Koos