There are many image processing programs that will allow what is termed 'layers'. I have Adobe Photoshop Elements but there are many others I am sure will
work as well. I just have not used others.
To use layers, you copy one coin image into the 'clipboard' and paste it over the other image creating a second layer. This will hide the coin underneath. In the upper right corner in Elements, there is a control that is labeled Opacity. Click on the arrow to open a slider that will allow you to reduce the transparency/opacity of the top coin so you can see through it to the coin below. Start around 50% but different coins may require a
bit more or less to show details best. You also will need to drag the image around to align the two. You may also need to resize or rotate a
bit if you did not shoot the two at exactly the same settings. A lot depends on how accurately the two images match so a little processing of the images may
help a lot. Of course this requires practice. This is easier to do after a few months of working with your individual
software. It is MUCH easier to do than to explain but not knowing what softwere you are using makes it impossible to give detailed instructions.