Hi Samuel,
Welcome to Microsoft Community.
Using a Surface Pro 8 as a secondary monitor for your Surface Pro 11 can be achieved using software-based solutions since neither USB-C nor Bluetooth directly supports extending displays. Here’s how you can set it up:
On the PC you'd like to project to:
- Select Start > Settings > System > Projecting to this PC .
- Under Add the “Wireless Display” optional feature to project to this PC, select Optional features.
- Next to Add an optional feature, select View features, then enter “wireless display.”
- Select the check box next to Wireless Display from the list of results, select Next, then select Install.
- After “Wireless Display” is installed, select Back to manage your projection preferences on the Projecting to this PC page in Settings.
- Launch the Wireless Display app by entering “wireless display” in the search box on the taskbar and select Wireless Display in the list of results.
On the PC you’re projecting from, select Windows logo key + K and select the PC you’d like to connect to.
Or you can search for Cast in the Windows Search bar, then select Connect to a wireless display. On the Display settings page, under Multiple displays, select Connect to a wireless display, then select Connect. Screen mirroring and projecting to your PC or wireless display - Microsoft Support
I hope this helps. If there is anything not clear, please do not hesitate to let me know.
Your Sincerely
Hahn. W - MSFT | Microsoft Community Support Specialist