2 x rx460 multimonitoring dont work

...running here the follow new system, multimonitoring dont work:

 

gigabyte board H270-HD3

32GB Ram

i7 proc

 

2 x Radeon Rx460

 

4 x display Dell U 2412 Ultrasharp   2 x DVI    2 x Displayport

 

Win 10 pro 64 bit

 

The problem is, only one of two grafikcards send signals out.

Both visible in system, but only the first gives signals (1DVI and 1DP).

The 3th and 4th display are black.

 

If I deinstall the second card and newinstall - while the drivers are loading - for a short moment (few seconds) all 4 displays working. If the driver install finish...still all the same - 1 card put signals out,the other card nothing.

 

Carddriver ist the latest form the site - 21.19.137.514

 

I need good Ideas, this is my office computer an I need all 4 displays....

 

thanks in advance

 

yakini