What you are describing is a project that is dependent on two packages, A and B. Package B can stand alone and be used in projects where Package A is not also used.

But, Package A requires Package B. If you want nuget to manage the dependency, you would put that dependency in the .nuspec file of Pacakage A. If you add Package B as a dependency in this way, you can go to nuget package manager, add Package A, and the nuget manager will automatically go and add Package B if it is not already installed in the project.

Here is a resource for .nuspec syntax. The ranges and wildcards for dependencies may be helpful if this is what you were looking for: nuspec reference

If you are manually managing the dependency, i.e. manually adding Package B, then you don't need to do anything in the packages themselves. You just have to install both packages in the projects where they are needed. This will create your csproj or packages.config entries.