If you can manage 2 40 hour per week jobs and they don't overlap, then no.
If you are working both of them ostensibly for the same hours, the regular M-F work week, then yes, you are stealing from the company as well as lying to them, as you are not giving them the time they think you are.
I find this an incredibly interesting thread to come back to and read through the comments.
I would say that the idea of work has radically changed due to the pandemic and technology.
I managed a workforce pre-pandemic that all work alone, get assigned clients to manage themselves. As long as the clients were taken care of very well and to standard/above, little interference from me in how they did their job.
Post-pandemic? I am seeing a lot of new hires thinking they can run their client based business and do ‘other things.’ Basically multi-task a job not fit for multi-tasking as we work with animals. We have changed software to address some of these issues that includes GPS tracking. A lot of people “think” they can manage independently, when really they need to be back in their office cubicle focused on the job at hand.