Is possible two gpus GT8600 and 1080Ti in one system?

I wanna to make GT8600 as primary GPU(for monitor connection) and make 21080Ti for working. Is it possible? Because how I can see in 390.26 specification, this driver not supports old GPUS such as GT. Installed driver 390.26 detected only Ti. But system detects all gpus(lspci | grep VGA), and shows me 21080Ti in 1,9 bus, and on 6 bus I can see GT8600. Plugged monitor works only with Ti. Can you tell me… how I can configure my Ubuntu x64 16.04 with three cards(1x8600GT and 2x1080Ti) and work with them? Is it possible really?)

You will need legacy driver for GT8600, that card is way too old to be supported by current drivers.
You probably have few options here to make this work, “simplest” being pcie passtrough virtualization to separate kernel with legacy driver and kernel with modern driver.

It might be possible to use both drivers (or nvidia for 1080ti and nouveau for older driver) at the same time and “bind” them per pcie slot (IOMMU), but I haven’t tried that.

echo "0000:01:00.0" > /sys/bus/pci/devices/0000:01:00.0/driver/unbind
echo "0000:01:00.0" > /sys/bus/pci/drivers/nouveau/bind

Well, assuming that nvidia driver won’t bind it because it’s way too old, you only need latter part I guess. You will have to do this manually I guess (blacklist nouveau, only use it for your older card).
Replace “0000:01:00.0” with pcie path to your 8600GT.
I’m not really positive, but I highly doubt that you can load both legacy and current nvidia drivers at the same time into the kernel, would probably cause some serious symbol mangling.

Considering that you have afforded 2x 1080Ti, I would honestly just recommend to save yourself the trouble and spend few bucks on GT 1030 or literally anything else to drive your “display” screen.