You will have three display adapters the one you are seeing in device manager is probably the Intel Gpu on the I7 cpu you bought.
Restart your machine, and enter the bios setup of it.
For the primary graphics display adapter interface change the mode set in the bios to Pci-e.
Save the changes in the bios before you exit it.
Now if you have the video lead connected to the motherboard display output ports.
Make sure you remove it and connect it to the First GTX 1080 Pci-e graphics card seated in the first Pci-e card slot of your motherboard.
The upper most card from the back of your system to the top of the tower case you have.
Windows 10 should then detect new graphics hardware, and begin searching for a driver automatically Jimbo.
If not go to here by clicking on the link in this post : http://www.geforce.com/drivers.
Select your type of card, the operating system you are running and also the bit version of windows 10 you have installed.
Download the latest Geforce driver for the GTX 1080 card, around 330Mb in size.
Once downloaded, run the program or Nvidia install and setup from where it saved the download file too.
Default path to save the file is C:\ windows\users\your name\downloads folder.
Ok so once the Nvidia driver is installed run launch the Nvidia suite for the GTX 1080 card.
Look through the options and select enable SLI.
NB: you must have the two cards linked via a SLI ribbon cable to the two Gtx 1080 cards fitted to the two Pci-e graphics card slots of your motherboard.
If the cable is not present, then fit one with the system turned off first of all.
If the SLI bridge ribbon cable is fitted then you should as said be able to enable sli mode in the Nvidia driver aplication software.
Just follow and check everything in this list as posted.
Just to double check one you installed the Nvidia Video drivers.
If you go into device manager of windows, and click on display adapters it should show two references to the GTX 1080 cards you have fitted to your system.
That proves they are working as they should, and the drivers are properly installed ok.