Yes, you can have a dual boot of your current macOS and macOS Catalina on a single Mac (without need for external SSD).
- Access to macOS Catalina
- Compatible Mac computer
- APFS-formatted disk
- At least 20GB of free space (if you want to play with Xcode 11 beta, be prepared to have about 50GB to spare)
- Open "Disk Utility" app
- Make sure your main volume is selected on the side panel
- Choose "Edit ➔ Add APFS Volume..." from app's menu
- "Add APFS Volume to container" popup should appear
- Name the new volume as you like (eg. "Catalina") and choose "APFS" format
- You can reserve the size for the new volume if you want ("Size options..." button), but you don't have to. By default, the new volume will share available space with your current volume.
- Download macOS Catalina installer (it should start automatically after downloaded)
- Choose newly created volume as an installation target
- Proceed with installation
Your Mac will restart several times and boot into fresh, new macOS Catalina installation. You will be able to choose the system on boot, just press and hold Option (⌥) key immediately after switching your Mac on. You can also change which system will boot automatically in "System Preferences ➔ Startup Disk".
Additionally, If you are using FileVault to encrypt your disk, macOS Catalina won't have access to your current macOS installation and all files you are storing with it until you provide the passphrase for FileVault. If you don't do this, you will keep your current files in isolation from the beta OS, which I recommend.
I am using the above setup to dual-boot macOS Mojave and Catalina without any issues on MacBook Pro 15", late 2016.
Please, remember to make a full backup of your system in advance - just in case.