While in multi-window mode, simply long-press an object (text or image) in one app and drag it to the other.
[**Example (copying text)**](https://i.imgur.com/l4CekmE.png)
This is very helpful when you don't want to tediously keep switching between two apps for copy-pasting data.
1. Copying image may/may not work depending on the apps.
2. Long tap & drag won't work if the app has some predefined action for long tap (for example, links in browsers)