How to extend screen on a secondary monitor in Android-x86?
A quick check of android-x86.org shows at the bottom:

Development (2019/10)
What we are working on now

Android 10 (Q release) porting (q-x86 branch)
Upgrade kernel to 4.19 or newer
OpenGL ES hardware acceleration for intel/radeon/nvidia/amdgpu (pstglia, mauro) and virgl(robh) GPUs.
HDMI audio (cwhuang)
LineageOS integration (jjm)
Hardware accelerated codecs

To-Do List

Multiple displays support
Better power management
Miracast


So the feature Multiple display support is on the To-Do List and probably dependent on OpenGL ES hardware acceleration efforts.
So as of 2019-10 the feature you want isn't available.