eVGA GeForce Titan X (Hybrid) + GeForce GTX-560Ti 2GB (second monitor - not PnP - stuff and Phys-X dedicated) - tested with the same hardware (HDD image for Window 8.1 and Windows 10):

• Windows 8.1 Pro 64bit: no problem at all, works like a charm (for months).

• Windows 10 (Pro) RTM 64bit: only a video card is active at all time (one, usually the GTX-560Ti is reported with the exclamation mark in Device Manager: "Windows has stopped this device because it has reported problems. (Code 43)"); if I remove from the Device Manager the GTX-560Ti, the eVGA GeForce Titan X will disappear(!!!) and the GTX-560Ti will became active (and works).

Tested with Windows 10 drivers: 353.30 WHQL and 353.54 WHQL (from Windows 10 Update). I tried all (first install a card, then the other and vice-versa). I hope nVidia will fix this problem really soon, in fact I want to use my 2 (nVidia) cards at the same time ;-]]]

If I remove from the device manager the 2 video cards (reboot) and do not install any drivers the GeForce GTX-560Ti 2GB is recognized as a Microsft Basic Display Adapter and always reported with the exclamation mark in Device Manager, with the error: "This device is not working properly because Windows cannot load the drivers required for this device. (Code 31). The driver trying to start is not the same as the driver for the POSTed display adapter.".
 
Internet rumors: "Windows 10 does not support multiple different gpu's right now".

But it's a problem of the nVidia drivers or a limitation of the Windows 10 OS, because I have read that with Windows 10 it's possible to mix GPU of different vendors and utilize them as a pool (with performance and sync degradation probably)... it's seem very strange to me I have utilized different GPU's from Windows XP (then Vista, 7, 8, 8.1) ATi/nVidia or only nVidia (but Always 2 different GPU's)... with GPU's of the same vendor the only limitations (in the past) is that the drivers must be the same, you can't mix different drivers revisions, and in Windows 10 the driver for the GeForce Titan X is the same of the GeForce GTX-560Ti. I would want an official reply from the official support if possible.

And then another question: if I substitute the GeForce GTX-560Ti with a GeForce GTX 960 (same Maxwell architecture of the GeForce Titan X) could the problem arise or it will work right now in Windows 10 (of course).

I repeat if possible I need an official answer from the driver/testing team because (I have not specified it before) I'm an hardware reseller and I do some configurations (for some special customers) with mixed GPU's, that in the past worked perfectly.

Thanks for the support in advance.