Can confirm this is an issue with SLI and running UE4 with -dx12. Only way I was able to fix it with my setup was simply disable SLI. Thankfully, not a total waste of the second card as I can (without SLI) run UE4 on card #1 and then run Blender and Photoshop on Card #2 (and chrome playing videos ). Allowing me to avoid running out of VRam. The entire computer seemed to be a bit more stable, as well, but that could just be a mind trick.
Its honestly kind of sad that nVidia (and AMD) have options for bridging but you can hardly do anything with it. Hardly anyone supports it and it appears even nVidia themselves don’t really do much to support it above just having the ability to enable it. My understanding is the technical aspect behind utilizing it is exponentially more complicated and presents limitations in what you can do with it. Even those who take advantage of it do really weird things to get it working and it’s mostly “hacky” ways to get it working.
So much for DX12 just natively taking advantages of both cards without needing to do anything with the SLI bridge (unless I’m misunderstanding something).