Hello !
What do you exactly mean by split-screen on your secondary monitor ?
If you want to disable the "Snap windows" feature that automatically splits the screen when you drag windows to edges, open Windows settings, go to "Multitasking" and switch "Snap windows" from "On" to "Off".
Windows will no longer snap when you drag them to the border on your screen.
Please let me know if you have any other question !