Currently, Windows does not support having Windows Spotlight on one monitor while using a custom wallpaper on another monitor simultaneously. Windows Spotlight is designed to provide a dynamic background experience, but it applies to the entire desktop environment rather than individual monitors.
If you want to have a specific wallpaper on one monitor and Windows Spotlight on another, you would need to set both monitors to the same background setting, either both to Spotlight or both to a custom wallpaper.
For now, you may consider providing feedback to Microsoft about this feature request, as user feedback can influence future updates and enhancements to Windows features.
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