https://www.vegascreativesoftware.info/us/forum/faq-graphics-cards-gpu-acceleration-for-vegas-pro--104614/
"VEGAS does not benefit from NVIDIA SLI or AMD CrossFire."
This was written in 2016, so I doubt it's changed (and that includes multi-GPU).
(they do say "SLI" and "Crossfire" but remember that's about producing a single output; it seems multi-GPU and SLI/Crossfire have become synonymous leading to some confusion. You can do multi-GPU without using SLI or Crossfire just like you can use more than one CPU core)
*Now here's an example of multi-GPU working:
https://www.pugetsystems.com/labs/articles/Adobe-Premiere-Pro-CC-2015-3-Pascal-GPU-Performance-840/#Exportingto4K-DualGPU
"you will notice that using two cards is rarely faster than using a single card. In fact, the only time it is faster is if you are using RED 6K footage. Even then, a single Titan X is as fast as any dual GPU configuration - even two Titan X's. Technically, two GTX 1070 cards would be cheaper than a single Titan X so there may be a cost reason to go with dual cards if you work with 6K+ footage, but at that resolution the additional VRAM on the Titan X is likely to be very useful. In addition, single GPU configurations tend to be more stable in the long term with fewer driver or software bugs popping up at random times."
Overall (currently) multi-gpu is not well supported. That's slowly shifting with OpenCL (not OpenGL) gaining traction but it's a very, very slow process as it's not trivial to implement.
So... ?
The 2nd GPU is best used with GAMES. Even then, if you play games, you should learn how to best use it as can cause issues like stuttering. In Batman Arkham Knight the best usage cause is PROBABLY to use the 2nd GPU as a dedicated PhysX card since it doesn't support SLI so that should help by over 30% at times.