How to uninstall completely system1/2?


What rom? Just go to system with proper rom and install MIUI from Updater. This will overwrite rom on second system.

Wysłane z mojego MI 3W przy użyciu Tapatalka
 
Just wipe data and cache for system 1 and you're good to go. And don't reboot to system 1 because it will add some data to unified data partition :)