What Ted said. Yes, Union has two layers plus a masking layer, and my go-to (because simple). Also as Ted says Diptic for panels/collages, some choice with colours and patterns. Autocollage is fun and has lots of choices for colour (including gradients). SketchClub is the best kept secret on the App Store as far as layering and overlapping is concerned.
GroovyGouvy and
Lzed are Mobi’s pre-eminent app experts.
ImageArt Starzee JillyG and I am probably the most
manic committed image collages/layer-ers — they and others may have some tips for you....
And very best wishes for your birthday!