Leonardo is from the same stable as Superimpose and can be described as its big brother. It supports all the layering, masking and blending capability you would expect of a specialist app capable of working with multiple active layers.
Leonardo is my go to iPhone photography app for working with more than two layers at the same time. It’s a 5 star app created by Pankaj Goswami who is also behind another of my favourite blending apps Superimpose.
Michael Orton started using this technique in the mid 1980s. His aim was to create a watercolor styled photograph by combining various captures of the same scene. The process is also known as Orton imagery and the Orton slide sandwich. An iPhoneography workflow.