How to utilize multiple GPUs?

I am using the github master branch of UE4 on Ubuntu. I have two GTX 970s in my machine, but UE4 seems to only use one. How do I specify that it use both of them? Alternatively, how do I specify that it use the other GPU, since currently it uses the one that supports the system graphics. Thanks!

Hello! Did you manage to get information about this?

Cheers!