A common complaint about multitasking in iOS 11 is that apps seemingly must reside in the Dock for them to be available for Slide Over or Split View. Here's Apple's description of multitasking on their iOS 11 preview page:
iOS 11 makes it easier and more intuitive than ever to multitask. You can open a second app right from the Dock, and both apps remain active in Slide Over as well as Split View. You can drag the second app in Slide Over to the left. And you can get back to your favorite App Spaces in the redesigned App Switcher.
As some iOS 11 users have pointed out, one alternative to this is to invoke Spotlight to search for apps, though this requires the use of an external keyboard to show Spotlight while an app is still active. Another option is to create a folder of apps and place it in the Dock, though this still means you're still limited to a selection of apps you can multitask from.
There is another way of multitasking apps that doesn't require using the Dock at all, allowing you to one-handedly drag any app from your Home screen and place them in Slide Over or Split View. You can even use this process to replace any app in a pairing.
- Press the Home button to go back to the Home screen
- Tap-and-hold an app until you can drag it around
- Tap to select another app and launch it from the Home screen
- Invoke the App Switcher (either by swiping up from the bottom of the screen or double-pressing the Home button)
- You can then drop the app in Slide Over or Split View, or replace either app in the pairing
It does feel clumsy at first and there's probably more Apple can do to improve this. But after using this technique many times already, it's become second nature and I can accomplish this all one-handed. I don't tend to use the app pairings that Apple is seemingly trying to push as a feature of iOS 11, instead just selecting apps and multitasking whenever necessary.
I've only been aware of these methods since using Beta 2 so cannot confirm whether it existed in the first beta or is a recent addition. Whether these are worthwhile solutions to app multitasking or just the first step in an ongoing improvement remains to be seen. But if this is how multitasking is going to work in the first public release of iOS 11, I'm fine with it for now.
Update: As reddit user dunnouniquename suggests, you can also drag an app from the Home screen with one hand then using another finger to open another app. This launches the app while you're still dragging the other one. I've updated the post and video to reflect both methods.