You can long-press an app icon with your index finger and then swipe up from the bottom of the screen with your thumb to open the app switcher - while still holding the first app icon with your index finger. Then drag the app under your index finger over the app you want to use in the app switcher until it selects the desired app in the app switcher. The app switcher app will then open full screen and you can continue to drag the app icon under your index finger to one side of the full-screen app. It sounds confusing, and it takes some practice, but it works well here on my iPad Air 3 with iPadOS 13.