if your dependency is a nuget package you can add a dependency tag in your nuspec file
<dependencies>
<group targetFramework=".NETCoreApp3.0" >
<dependency id="myproject.Interfaces" version="3.0.1" />
<dependency id="myproject.Models" version="3.0.1" />
</group>
<group targetFramework=".NETCoreApp3.1" >
<dependency id="myproject.Interfaces" version="3.0.1" />
<dependency id="myproject.Models" version="3.0.1" />
</group>
</dependencies>
but if your dependency is a dll you can add as a normal file
<files>
<file src="lib\netcoreapp3.0\myproject.Interfaces.dll" target="lib\netcoreapp3.0\myproject.Interfaces.dll" />
<file src="lib\netcoreapp3.0\myproject.Models.dll" target="lib\netcoreapp3.0\myproject.Models.dll" />
<file src="lib\netcoreapp3.1\myproject.Interfaces.dll" target="lib\netcoreapp3.1\myproject.Interfaces.dll" />
<file src="lib\netcoreapp3.1\myproject.Models.dll" target="lib\netcoreapp3.1\myproject.Models.dll" />
</files>