[DllImport("test")]
public static extern int Foo();

using System.Runtime.InteropServices;
namespace DualLibClassLibrary
{
    public static class Class
    {
        [DllImport("test")]
        public static extern int Foo();
        public static int Bar()
        {
            return Foo() * 1000;
        }
    }
}

int Foo() { return 123; }

int Foo() { return 456; }

New-Item "test.c" -ItemType File -Value "int Foo() { return 123; }"

gcc -v -c test.c
gcc -v -shared -o test.dll test.o

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFrameworks>netstandard2.0;net45</TargetFrameworks>
    <LangVersion>6</LangVersion>
    <Configurations>Debug;Release</Configurations>
  </PropertyGroup>
  <PropertyGroup>
    <PackageId>DualLibClassLibrary</PackageId>
    <Version>1.0.0</Version>
    <Authors>melanchall</Authors>
    <Owners>melanchall</Owners>
    <Description>Dual-lib class library</Description>
    <Copyright>Copyright ​ Melanchall 2021</Copyright>
    <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
  </PropertyGroup>
  <ItemGroup>
    <Content Include="test.dll">
      <Pack>true</Pack>
      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
    </Content>
    <Content Include="test.dylib">
      <Pack>true</Pack>
      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
    </Content>
  </ItemGroup>
</Project>

static void Main(string[] args)
{
    var result = DualLibClassLibrary.Class.Bar();
    Console.WriteLine($"Result = {result}. Press any key to exit...");
    Console.ReadKey();
}

<?xml version="1.0" encoding="utf-8"?>
<package xmlns="http://schemas.microsoft.com/packaging/2012/06/nuspec.xsd">
  <metadata>
    <id>DualLibClassLibrary</id>
    <version>1.0.0</version>
    <authors>melanchall</authors>
    <owners></owners>
    <requireLicenseAcceptance>false</requireLicenseAcceptance>
    <description>Dual-lib class library</description>
    <copyright>Copyright ​ Melanchall 2021</copyright>
    <dependencies>
      <group targetFramework=".NETFramework4.5" />
      <group targetFramework=".NETStandard2.0" />
    </dependencies>
    <contentFiles>
      <files include="any/net45/test.dll" buildAction="Content" />
      <files include="any/netstandard2.0/test.dll" buildAction="Content" />
      <files include="any/net45/test.dylib" buildAction="Content" />
      <files include="any/netstandard2.0/test.dylib" buildAction="Content" />
    </contentFiles>
  </metadata>
</package>

<Content Include="test.dll">
  <Pack>true</Pack>
  <CopyToOutputDirectory>Always</CopyToOutputDirectory>
  <PackageCopyToOutput>true</PackageCopyToOutput>
  <PackagePath>contentFiles;content</PackagePath>
</Content>
<Content Include="test.dylib">
  <Pack>true</Pack>
  <CopyToOutputDirectory>Always</CopyToOutputDirectory>
  <PackageCopyToOutput>true</PackageCopyToOutput>
  <PackagePath>contentFiles;content</PackagePath>
</Content>

<?xml version="1.0" encoding="utf-8"?>
<package xmlns="http://schemas.microsoft.com/packaging/2012/06/nuspec.xsd">
  <metadata>
    <id>DualLibClassLibrary</id>
    <version>1.0.1</version>
    <authors>melanchall</authors>
    <owners></owners>
    <requireLicenseAcceptance>false</requireLicenseAcceptance>
    <description>Dual-lib class library</description>
    <copyright>Copyright ​ Melanchall 2021</copyright>
    <dependencies>
      <group targetFramework=".NETFramework4.5" />
      <group targetFramework=".NETStandard2.0" />
    </dependencies>
    <contentFiles>
      <files include="test.dll" buildAction="Content" copyToOutput="true" />
      <files include="test.dylib" buildAction="Content" copyToOutput="true" />
    </contentFiles>
  </metadata>
</package>

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <ItemGroup>
    <None Include="$(MSBuildThisFileDirectory)test.dll">
      <Link>test.dll</Link>
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
    </None>
    <None Include="$(MSBuildThisFileDirectory)test.dylib">
      <Link>test.dylib</Link>
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
    </None>
  </ItemGroup>
</Project>

<ItemGroup>
  <None Include="test.dll">
    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
    <PackagePath>build\</PackagePath>
    <Pack>true</Pack>
  </None>
  <None Include="test.dylib">
    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
    <PackagePath>build\</PackagePath>
    <Pack>true</Pack>
  </None>
  <None Include="DualLibClassLibrary.targets">
    <PackagePath>build\</PackagePath>
    <Pack>true</Pack>
  </None>
</ItemGroup>

Write-Host "Downloading winlibs..."
Invoke-WebRequest -Uri "https://github.com/brechtsanders/winlibs_mingw/releases/download/11.1.0-12.0.0-9.0.0-r1/winlibs-i686-posix-dwarf-gcc-11.1.0-mingw-w64-9.0.0-r1.zip" -OutFile "winlibs.zip"
Write-Host "Downloaded."
Write-Host "Extracting winlibs..."
Expand-Archive -LiteralPath 'winlibs.zip' -DestinationPath "winlibs"
Write-Host "Extracted."
Write-Host "Building DLL..."
$gccPath = Get-ChildItem -Path "winlibs" -File -Filter "i686-w64-mingw32-gcc.exe" -Recurse
& $gccPath.FullName -c test.c -m32
& $gccPath.FullName -shared -o test.dll test.o -m32
Write-Host "Built."

namespace DualLibClassLibrary
{
    internal abstract class Api
    {
        public abstract int Method();
    }
}

using System.Runtime.InteropServices;
namespace DualLibClassLibrary
{
    internal sealed class Api32 : Api
    {
        [DllImport("test32")]
        public static extern int Foo();
        public override int Method()
        {
            return Foo();
        }
    }
}

using System.Runtime.InteropServices;
namespace DualLibClassLibrary
{
    internal sealed class Api64 : Api
    {
        [DllImport("test64")]
        public static extern int Foo();
        public override int Method()
        {
            return Foo();
        }
    }
}

using System;
namespace DualLibClassLibrary
{
    internal static class ApiProvider
    {
        private static readonly bool Is64Bit = IntPtr.Size == 8;
        private static Api _api;
        public static Api Api
        {
            get
            {
                if (_api == null)
                    _api = Is64Bit ? (Api)new Api64() : new Api32();
                return _api;
            }
        }
    }
}

namespace DualLibClassLibrary
{
    public static class Class
    {
        public static int Bar()
        {
            return ApiProvider.Api.Method() * 1000;
        }
    }
}