Hello Swedenlo,
Thanks for asking your question here in the Apple Support Communities. I'm happy to offer some info about Slide Over.
The help article from Apple is here: About Multitasking on your iPhone, iPad, and iPod touch. It describes how to use Slide Over, and some other multitasking options on your iPad. There does not seem to be a way to modify the apps listed in Slide Over, but I did notice that it puts the last used app up to the front of the list. As a work around, you may wish to open the app you wish to be positioned at the front of the list for Slide Over, then open the other app you wish to work in. Whenever you need, activate Slide Over, and the last app will be listed there at the front of the list.
Thanks again for bringing up your question here in the Apple Support Communities. Have a great rest of your day.