How to extend desktop across second mac

I have 2 Macs on my desk: An M1 Macbook Air, and an M1 Mac mini.

What I want to do is display my Macbook air in EXTENDED DESKTOP, on My Mac Mimi's Ultrawide Monitor. 

Currently, I can connect to the Macbook from the Mimi via the Network and "Share Screen". This displays the Macbook's screen on my Mini's Ultrawide monitor, but it's not all that I want. I want to be able to do that AND display an Extended Desktop at the same time (Showing 2 x Macbook Air displays on on Mini's Ultrawide). Displaying the MacBook Air's single display on my big monitor is nice, but doesn't give me the extra desktop space I need. 

Does anyone have any ideas on how to acomplish this?