IIRC, SLI support in UE4 requires using NVidia’s custom source build of the engine (outdated). I think there are now somewhat-different solutions available in DX12 and Vulkan now for “mGPU” use, but that likely would also require modifications of UE4 engine source code. I give it about a 40% chance of SLI or some other mGPU solution being supported natively in UE5. Maybe more like 25% chance of having the extreme features like doubled VRAM.
Modeling programs like Blender are much more likely to support NVLink’s memory doubling capabilities.
I’ve seen “Nvidia driver has crashed and is restarting” errors that I don’t see any reason for besides very low performance, before. I bet upgrading to an RTX 2080 or better (3070, 3080, 3090, maybe the next top Radeon card if it supports DXR) will resolve those. SLI/NVLink support for non-offline-rendering applications is questionable at the moment from NVidia’s statement as passing the torch to individual developers. The only new card with NVLink is the 3090 anyway, so if you absolutely must have the best performance, you have nothing to lose if you buy a 3090 now and wait to see if NVLink SLI support is common.
Another factor here is the somewhat-confirmed unreleased 20GB VRAM 3080.