I considered doing this with my last job. I was the sole sysadmin/infrastructure/devops engineer there, but still had almost nothing to do 90% of the time. I doubt I worked more than 16 hours a week on average, probably less than 8 tbh. I got called for emergencies maybe once every couple of months, most of which took less than an hour to resolve. At least half of of those would have taken even less time if the developers hadn't tried to fix things on their own before calling me (my hours were radically different from everyone else)
Working a second job would have had no impact on the company at all, especially if it was a similar workload. I doubt they would have ever noticed. The only reason I didn't is because I hate job searching with a passion.
Sadly they decided to fire the entire technical team (except for me!) a couple of months ago and the work I would have been doing afterwards was decidedly uninteresting, so I've moved on.
I'm not sure how things will be at my new company yet, but so far it doesn't seem like it will be much different, but I'm not fully onboarded yet either. If I was just doing operations work, I think it would be fairly simple to balance another job. It seems like most of the issues that have popped up so far would be easy to automate away.