One of my video editing programs had a transition like this (Polaroid border instead of equal border) but was not very smooth if you increase the transition time. I thought I could make something like it in PTE and so here it is. It uses a parent child mask to create the white border around the images. The background rectangle is set to 0% so you can use a background splitter object and your own images. It was a fun learning experience.
Thanks,
Tom
Snapshot.pteeff