This is a coördination problem: the municipality can mandate that every company has to install a larger conduit so that (say) 3-6 (?) other fibres can be run.
So if one company is doing the east part of the town, and another company is doing the west, at some point they could leverage the infrastructure of each other.
I don't think so. German open access law says if you're in a "market dominating" position, you have to share access to what you have. I'm at least unaware of a law that allows forcing telcos to build more than they want. (And even if there were, it'd be balanced by them deciding to just not build at all.)
So if one company is doing the east part of the town, and another company is doing the west, at some point they could leverage the infrastructure of each other.