FromEvent.cs

public class Hoge
{
    public Action<int, string> onSomethingWithArgs2;

    public void Hoge2()
    {
        var source = Observable.FromEvent<Action<int, string>, Tuple<int, string>>(
            h => (x, y) => h(Tuple.Create(x, y)),
            h => onSomethingWithArgs2 += h,
            h => onSomethingWithArgs2 -= h);

        source.Select(x => x.Item1 + x.Item2).Subscribe();
    }
}