That's the driver update which came out a few days ago. Since "upgrading," I see that Visualize is only making use of a single GPU. It was working beautifully before this. When I rolled back to 391.35, Visualize started using the other GPU just as before.
I'm guessing the fix is as simple as having to tweak a setting somewhere, but I figured I'd ask to see if anyone else has run into the issue first.
The setup is dual Titan XPs with Win10 and Visualize 2018 SP2.