Why is MacBook Air trackpad spanning to completely separate system?

Here's a new one on me.... - MacBookAir, M1, OS15.1.1 (Sequoia) - 2019 MacPro, Intel, OS15.1.1 (Sequoia) The two systems are **not** directly connected, although they are both on the same wifi network, using the same Apple ID, and about 4 feet apart. Using the trackpad on the MBA, the cursor will *span* to the monitor on the 2019 MacPro. Note: *not* as an "external monitor" on the MBA, but as a **direct controller** of the MacPro. I can move the MBA's trackpad cursor to the MacPro screen and click, drag, etc on the MacPro, altering the MacPro. If the MBA's cursor is on the screen of the MacPro, then the MBA's keyboard also alters the MacPro directly. Anyone know what's going on?? Screen sharing, Remote LogIn, etc. is all off on both systems. File sharing is the only thing enabled on the systems.