This is a utility (NupkgMerge.exe) enables you to combine two NuGet packages into a single package (Multi-TargetFrameworks).
Usage
NupkgMerge.exe -P[rimary] "primary.nupkg" -S[econd] "second.nupkg" -O[ut] "output.nupkg"
For the convenience of building your packages, you can also install the NuGet Packages Merge Tool from NuGet NupkgMerge:
PM> Install-Package NupkgMerge