I have here some example code (below) that I believe should work fine.
However, I get the error

Error    1    The call is ambiguous between the following methods or
properties:
'TestExtensions.ExtensionsOne.AddFive<TestExtensions.One>(TestExtensions.One)'
and
'TestExtensions.ExtensionsTwo.AddFive<TestExtensions.One>(TestExtensions.One)'
C:\workspace\TestProjects\TestExtensions\TestExtensions\Program.cs
44    4    TestExtensions

It's as though the resolution of the extension functions ignores the
where clause on the functions. As a workaround I can call the function
statically. I just think this should be resolvable.

namespace TestExtensions
{
    public interface IOne
    {
        int Int { get; set; }
    }

    public class One: IOne
    {
        public int Int { get; set; }
    }

    public interface ITwo
    {
        int Int { get; set; }
    }

    public class Two : IOne
    {
        public int Int { get; set; }
    }

    public static class ExtensionsOne
    {
        public static void AddFive<T>(this T obj) where T : IOne
        {
            obj.Int += 5;
        }
    }

    public static class ExtensionsTwo
    {
        public static void AddFive<T>(this T obj) where T : ITwo
        {
            obj.Int += 5;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var one = new One();
            one.AddFive();
        }
    }
}