gistfile1.cs

public interface IFoo
{
    [Sql("select 1")]
    int GetNumber();
}

public interface IBar
{
    [Sql("select 2")]
    int GetNumber();
}

public interface IQux : IFoo, IBar
{
    [Sql("select 'Test'")]
    string GetString();
}

// later...

[Test]
public void Test1()
{
    using (var conn = GetConnection())
    {
        var db = conn.As<IQux>();
        var n1 = (db as IFoo).GetNumber(); // 1
        var n2 = (db as IBar).GetNumber(); // 1
        var s = db.GetString(); // "Test"
    }
}