Dirk your situation could still be done in the manner suggested.
Option 1
You could treat each pipe cl as an alignment and get volumes for the trench section
for each pipe
This could be done using either offsets within the assembly.
Option 2
Just use generic links.
I say use generic links, as they have the ability to be assigned different, and unique Point, and Link, Codes
These allow you to then create all the surfaces required to get all volumes for trenching and back fill material(s)
The reason I would NOT tend to use assembly from polyline, is the process to assign point link and shape codes annoys me.