Texture baking with two different GPUs

Hello all,

I’m an architect doing ArchViz with Unity and Blender. Most of what I model I texture bake in Blender. I’ve found that texture baking takes a very long time compared to regular rendering, and so in order to speed up the process, I have purchased a second GPU. Here’s what I’ve got.

EVGA GeForce GTX 1080 ti 11GB Hybrid

ASUS GeForce GTX 1080 8GB

Both of these are very fast for normal rendering, but take forever to bake textures. What’s really puzzling is that running them both at the same time actually takes longer than running the 1080 ti by itself. I don’t understand why, but I’ve notice that when both GPUs are selected for baking, the faster 1080 ti spends a lot of time idle, while the regular 1080 keeps running. I assume they’re dividing up the tiles, but it seems that the regular 1080 takes much longer/tile than it would running alone. I’ve had the ASUS for about three months, so I’m pretty familiar with how long it takes to bake a texture.

I’ve also tried simply running two instances of Blender, but I can’t, as two instances max out my computer’s ram, which is also a surprise, since I thought GPU rendering meant most of the work happens in vram.

Any ideas? Here’s the rest of my system info.

AMD Ryzen 1700
Gskill Trident Z dram (16gb)
MSI X370 Gaming Pro
Blender 2.79

Also, I’m using BakeTool to bake 8192 x 8192 textures with 500 samples. It’s surprising how much noise I still get from that many samples at that size, but I understand that denoising doesn’t work with texture baking. I’ve tried using the regular baking process, too, and it’s about the same amount of time for the same results.