They would be independent devices and not able to work together like that. I don't see any purpose to that really unless you wanted to drive a LOT of displays.
SLI requires the two cards to be physically connected with a separate connector that connects to the sides of the cards called an SLI bridge and reside on the same PCIe bus.
The purpose would be because a 980ti is not adequate for 4k 60 fps in these most modern games, for example, Dead by Daylight, i get like 45 fps. Also, Rainbow Six Siege, i get around 60, but drops to 40-30. So thats why i was saying, im pretty sure you can do something to get it to work, or maybe just put then close together and put on your sli bridge? Or get a motherboard, do something to get the alienware adapter there, then put both GPU's and a Bridge?
I don't think that would work for a whole host of reasons - the riser isn't a standard motherboard, there would be signaling differences between thunderbolt and the native PCIe that would make SLI not able to sync, etc. Get a 1080 and the AGA and that's the best you can do. If you really need 60fps 4k at max everything, you are going to need a desktop with 1080 SLI to do it.