The reason is the same as fort the fact that you cannot move the mouse between screens.

This stuff is not yet implemented unlike when using separate graphic cards. MorphOS sees dual head monitors as a stack of screens, while using two gfx cards result in two separate monitors

Currently the active screen is the front screen and that one gets a blanker. There is nothing you can do about it beside waiting for the implementation to get completed in the future.

[ Edited by geit 13.11.2019 - 22:49 ]