How to set FULLSCREEN on multiple Displays??

Hi!

Hopefully someone can help me.
I am developing a multi-monitor application with Unity. (The monitors are 4k but i want the application to also display in full-screen when i configure a smaller resolution)

Using multiple cameras and setting the targetDisplay variable, I can successfully output to multiple monitors.
However, when using a rendering resolution smaller than the displays resolution, i am not able to get a full-screen image on my secondary monitor.

Screen.fullScreen = true sets the first Display to Fullscreen but the second one stays a boxed Window.
I am unable to access a full-screen property for the other displays.

Please, if somebody has a suggestion or had a similar problem, i would be super happy for some advise.

Thank you, Cheers!
Martin