I have 2 solutions:
When I build Solution2 it fails because nuget pulls the NU1 package to the packages of Solution2, and when Project1.1 is built as a dependency, the package NU1 is missing.
Prior questions and answers have convinced me to reference projects better that .dlls -- however, I am afraid I have not yet understood how to follow this guideline when working with nuget.
Thank you in advance for your comments!
Eventually, I managed like this:
=> all solutions use same packages :)