I dunno about iOS, but there's that split screen thing on Android, if you haven't used it.
kagis
https://www.groovypost.com/howto/split-screen-on-android/
I've found it to be of limited use on phones, because the screen is small to start with, but I've used it on Android tablets.
If you don't care about the phone form factor (or don't care about power usage on the existing hardware and can do GNU/Linux on it), you can run Android apps on Linux on Waydroid, though I've never heard of someone doing it on a phone, myself. I don't know if resource usage makes that impractical.