The cause of the issue would be the way how the displays are connected to the system.
Using two video controllers from different manufacturers in the same system (Intel+ATI) is called switchable graphics. This allows you to get the processing benefits from one or the other.
However, for this configuration you physically connect the displays to one or the other and a third party software bypass the video processing between one or the other. So, this configuration is not designed to be using the two video cards actively and physically at the same time, even though it's working for you, this seems to be not supported by the system you are using.