I think you are looking for the QuantumOperator method. The following example should make your image 50% transparent.
After that you should be able to combine both your images using the correct CompositeOperator. You probably have to experiment a bit to find the right one.
using (MagickImage image = new MagickImage("input.png")) { image.Alpha(AlphaOption.Set); // If your image does not contain an alpha channel. image.QuantumOperator(Channels.Alpha, EvaluateOperator.Set, Quantum.Max / 2); image.Write("output.png"); }