Hy,
go to element attributes->composites and selct your sandwich wall.
After the skin list button there is a check box for the separator line and then the line pen to be used.
Hi,
this is not a pen problem but an intersection problem.
The inner stratus is weaker than the outer pannel.
Go to building material and change the intersection priority of one of the two materials to make it work properly. (Perhaps it's best to duplicate the inner material and youse this clone to give it a greater priority).