Applications: Why Android can support 2 applications in split screen but not in the background?

I read another question in Ask Different about why applications have to be reloaded when you switch to another application and you come back, and I think it's the same for Android.

But why does not this happen on the split screen?

For example, when I am using an application that consumes resources (pokémon Go, for example), if I change to another application and I stay too long with it, the system will eliminate other applications (possibly the first one that consumes more), and when I return to the game, I'll have to face the (long) load again.

However, I can divide the screen while I play, have the game in one half and any other application in the other.

  • Why is it so different? If the system is capable of running Youtube and Pokémon Go at the same time, why does it kill Pokémon Go if I do a single search on the YouTube application when it is not split?

  • If the system "locks up" Pokémon Go and kills other applications when it is split, can not I "lock" it in normal mode as well?