Hello Jochen,
Please feel welcome to ask Microsoft or Intel how "cloning across graphics adapters" works.
You can also look at the slide 4 of the following presentation not made by DisplayLink and not for DisplayLink so it will rule out any bias: "
Graphics Investments in Windows 10" Cross-adapter resource sharing was brought as optional in WDDM 1.3. You will see that "DWM Clone" is compulsory in WDDM 2.0.
Slide 7 even states that there is "no cloning across multiple GPUs", which is not totally correct. It was only optional which means most GPU vendors didn't implement it, but it was possible as described in slide 4.
Your driver says in its very title it is WDDM 1.1.
On the Windows 10 Anniversary Update our driver does NOT talk with the Intel graphics driver directly anymore, ever, and it does not have to. We uses Microsoft documented OS calls.
The reason I was able to reply quickly is because we did study it in details, with Microsoft, before Windows 10 Anniversary Update was released.
The ultimate proof? Remove the Intel driver and suddenly you'll be able to clone because the Microsoft inboxed driver is WDDM 2.0 so support DWM Clone.
Regards,
Alban