What to watch out for when running two instances of Hashcat at once?

[royce]

I suppose if you're managing a good cracking rig this would make sense. I never thought of that, you got me there. Only having a single card, I wouldn't want to be running multiple instances at once.

Generally true; the only corner case is when you have two very inefficient attacks (that you still have to run for some reason). The GPUs will happily handle multiple such jobs.