Your code:
Foo\
Foo.cs
Foo.Android.cs
Foo.iOS.cs
In your.csproj
<PropertyGroup>
...........
<!-- change this to iOS -->
<OS>Android</OS>
</PropertyGroup>
<ItemGroup Condition="$(OS) != 'Android'">
<Compile Remove="**/*.Android.cs" />
<None Include="**/*.Android.cs" />
</ItemGroup>
<ItemGroup Condition="$(OS) != 'iOS'">
<Compile Remove="**/*.iOS.cs" />
<None Include="**/*.iOS.cs" />
</ItemGroup>