I’m not sure if this is even launcher dependant thing? I’m on pixel 9 pro, using lawnchair myself.

In your open app-selector, you should have drop-down to start split screen mode.

I must mention this FOSS app! https://github.com/danilkinkin/buckwheat

I use this to track my monthly spend with my cards that I manually swipe at stores. This is the only budgeting app that has stuck with me for quite some time.