As a callback to the old Ace Doubles, I used the first parts of Vail’s World and Georjet of Mars to make my own double: Dr.Occulari’s Double #1.
Here’s the Georjet cover:

Flip the book, and read Vail’s World:

I used Affinity Designer to lay out the covers, Affinity Publisher for the interior layouts, and Procreate on my iPad to create the rather crude cherry blossom art for the Georjet cover.
The tricky part was reversing the pdf page order for one of the stories without having to drag and drop one page at a time. Searching (DuckDuckGoing; not Googling) turned up an online tool, but I don’t know anything about the developer. I eventually looked into the Mac OS Automator, and after a half hour of tweaking and testing I had a useable, and useful PDF page order reversing custom workflow.
I used the Lulu print-on-demand service to proof and produce a few sample books — booklets, really. They’re not for sale; I want a few hard copies to hand out to friends.