Recently I have built a new workstation PC with dual GTX 1080 Ti. I know that in Cycles, each GPU renders 1 tile but I'm not sure about EEVEE. I tested it and it seems that even if I have checked both GPUs in user preferences EEVEE still uses only 1 GPU. I tried opening 2 instances of Blender 2.8 and set different GPU in each but the result was same; it keeps using one GPU only. This might not be implemented yet or SLI bridge might be required, I am really not sure. Do you guys have any experience with it? Has anyone done some rendering using SLI?
Multi GPU support for EEVEE is a really hard goal to atchieve because of the nature of the technology. It is not being worked on at the moment and it is not planned for any time soon. EEVEE might benefit from SLI or Crossfire setups. It would be logical to wait untill it's realeased to test that though. I believe it is safe to assume that EEVEE will definitely not be able to use multiple GPUs that are not connected with SLI/Crossfire for real time rendering when it is released or any time soon after.
First of all, the Preferences settings where you can select which devices to use is a Cycles render setting only. There are no settings in Blender that will affect which GPU(s) Eevee is using. That depends entirely on which display Blender is started/active on.
It's possible to play games (on Windows at least) where you start one copy of Blender, then change the default OpenGL GPU to a second one and start a second instance of Blender and then you have two copies of Blender each with a single dedicated card for Eevee so you can run two render jobs in parallel for increased throughput.
Eevee does not currently (and may never) know how to utilize separate GPU devices the way Cycles can. However in SLI mode, two video cards basically become one, and I don't know for sure if SLI will currently affect Eevee performance (lots of people think it won't because "Eevee can't use more than one GPU" but I haven't found anyone who has definitively tested it!).
If you do have two cards in SLI mode, then I believe Cycles will not see them as distinct devices and won't be able to use both of them, which is a big downside even if SLI works (now or in the future) with Eevee.
@Zoot, I can confirm that your last statement is not true; Even wth 2 cards in SLI, both are used in Cycles (2 tiles). SLI is ignored here. Not tested if Eevee benefits from SLI though.