I am running an iMac with 8GB GPU AMD Radeon Pro 680, but I also have an 8GB Sonnet Breakaway Box via Thunderbolt that would be very helpful to utilize. I know Davinci Resolve is able to do this, and it’s massively helpful.
Currently in UE4 it’s not being utilized at all… see the GPU usage window in the lower right of this screenshot…
I know Davinci Resolve is able to do
this, and it’s massively helpful.
Judging by this somewhat in-depth article, this is not the case. While it supports multiple eGPUs via OpenCl, mixing integrated and external is not an option:
Just fyi the article you posted above says in the first paragraph:
“Even though Final Cut Pro X curiously doesn’t support external GPUs yet, DaVinci Resolve is another popular NLE that already works with eGPUs on macOS. In fact, the $299 Studio edition supports multiple GPUs, which can have a noticeable effect on both timeline and render/export performance.”
But to see it in action, here’s a screen shot of it running some 4k footage. But as mentioned, I had to purchase the studio version to enable that benefit. (Sorry it’s sideways, can’t figure out why it’s rotating after I upload it)
I hear what you’re saying that it’s a long ways off UE4 having this sort of achievement for free software, and on a Mac platform no less, I just was hoping it would also be a feature I could utilize given my ambitions with this new toy 
There’s never been any support for multiple GPUs in the editor, even for the humble SLI / Crossfire. And here we’re talking about 2 different GPUs and on top of that one is integrated and the other external GPU, and both on a platform that is not all that well supported in the first place.
Sadly, I do not see this happening.
I believe that the closest we’ve ever gotten to this was in older PhysX games where a single nVidia card could be dedicated to physics calculations. And it actually worked well.
I know Davinci Resolve is able to do
this, and it’s massively helpful.
I had a quick peek at:
But could not see any mentions regarding utilisation of 2 different GPUs (I skimmed it only). Do you, perhaps, know any sources that show how they pull it off?
It supports multiple GPUs but does not support a combination of integrated and external. In the pic you see 2x 580s, not a combo of integrated + eGPU.
I am assuming here that both are external cards here. Or am I mistaken?
I just was hoping it would also be a
feature I could utilize given my
ambitions with this new toy
I used to run both crossfire and sli, and even had a dedicated physics card at some point. But the software support was generally lacking. Switched back to a single powerful GPU and never looked back. 
Would love to see some proper dedication to making it work to be honest.
But nah, historically UE4 has not supported multiple gpus. I believe the sli / crossfire support has to be worked into the final product by the developer.
I wouldn’t mind having a couple of extra eGPUs waiting to kick in only when I need some parallel number crunching backup.
Yeah that’s my internal on the bottom and on top is my Sonnet Breakaway Box.
I will have to make the jump to something more powerful eventually, but for the time being the eGPU is such a useful bridge; its the difference between having to offline footage or edit 4k natively, such a quicker workflow in other programs. Hint hint, UE4…
