Go to Toad main menu, Help | Tile Vertical, and the Editor and the Schema Browser Window will be visible, both at the same time
Go to the Editor window and click on the restore button in the upper right corner between the minimize and maximize button, and then go Schema Browser and click on the restore button too, this way both windows will be visible at the same time.
In addition to keep open the Editor and the Schema Browser, these options are available at the same time,
1. Project manager
2. SQL Recall
3. Code Snippets
4. Object Palette
Please see the image below: