Hey guys,
I also just got my new S21 and this was a functionality I sorely missed, coming from an LG K11+ (a much simpler, low-end phone). Naturally, I wasn't about to just accept the fact that they simply made the process 100x more sluggish for no reason, so I investigated and I actually found out the solution. I'm hoping this post can help more people 
Apparently, what they decided is that the new 'Quick Apps Sidebar' would be the hub for the multitasking. When in split-screen mode OR in normal 1-app mode, open up the sidebar and choose the app you want to open. If you don't see it, you can tap the last item in the sidebar, which is just an image of three lines, and then choose "All apps". When you've chosen your app of choice, hold it and drag it to a part of the screen (top, middle or bottom). Releasing it in the top or bottom of the screen will open them as the multitask app in the desired slot (top or bottom), and releasing it in the middle of the screen will open it in Popup mode.
If you basically always open the apps in the bottom slot, you can even, in the sidebar, tap the last 'three lines' item, tap "Edit", then tap the three dot options in the upper right corner, and change the "Open in split screen view" option to "Tap". This means that, when you just tap any app in the sidebar, it'll default to opening up as your bottom app in multitasking mode. Cool thing is, the drag-n-drop functionality still remains, even in this mode.
This is definitely a very different way of handling things. On one hand, they did solve the fact that, to enter split screen mode, you don't need to exit the app at all, you can do it seamlessly. On the other hand, it takes away from the fact that to chose your bottom app in the old way, you'd navigate through your home app, which just felt a lot more comfortable since you do that all the time. It'll definitely take some getting used to.
I hope you guys read this post, as it's something I'd definitely want to know if I hadn't figured it out by myself!
