Let me tell you the quirkiest story about an “entrepreneurial” academic who managed to get himself hired into two tenure-track positions at the same time, at two different higher education institutions. The schools were only about one-hour drive from each other. So, our scholar found a house in the middle and commuted to both!
He was hired as an ABD (All But Dissertation), which is the designation that means the person has completed all the other requirements of a doctoral program, i.e. coursework and comprehensive exams, as well as made good progress on the dissertation, and was left only with submitting and defending it. It usually takes a semester or two between ABD and PhD and most doctoral candidates start job search and interviews as ABDs. Colleges and universities hire them if they show good progress and potential of defending their dissertations within a year.
So, this doctoral student was hired with an ABD at both schools as a tenure-track assistant professor and started teaching and working on finishing his dissertation.
Anyone in academia knows what a tenure-track position involves. You basically work your butt off, putting your best foot forward: constantly improving the quality of your teaching based on student evaluations and peer feedback; staying current in your field; advising students on their college advancement and future careers; conducting original research and developing professionally by participating in various professional associations and attending their conferences; serving on various committees both at the school and in the professional organizations; engaging with the larger community, etc. All in the hope of securing a permanent employment with your institution by getting tenured and becoming an associate professor. Usually, it takes 5-7 years to get the tenure promotion under normal circumstances.
Imagine carrying the double of that load at the same time at two different schools, which our assistant professor was attempting!
Obviously, no one can do it well. The demands on time and commitment in academia are so high that he inevitably started not showing up for classes with conflicting schedules, skipped faculty meetings and under-served on the committees he’d been elected to or volunteered for, and was generally absent a lot, failing in many areas.
Everyone in academia is extremely busy. Even your next-door colleagues would not pay attention to your schedule, respecting each other’s autonomy and trusting in each other’s ethics. So, this type of behavior can indeed continue for some time.
At some point, students started to complain about our academic not showing up for classes or cancelling them at the last moment. He was brazen enough to explain it away by pointing out that he was still working on his dissertation and didn’t have enough time for everything. Newbies in academia get a lot of empathy from their colleagues: One of the institutions was so accommodating as to give him a room at the library where he could “escape” to work on his dissertation because the students were bothering him too much in his office.
This splitting of time and commitment between the two schools and skating off responsibilities continued for several years! The irony is that our moonlighter was teaching a course in white-collar crime at one of those institutions.
Again, two full-time jobs in academia is not an easy feat. You cannot split yourself into two people at the same time and will inevitably prioritize one school over the other. If you prioritize whole-heartedly, you may get tenured by the prioritized school earlier than by the second one.
Your students and colleagues at the neglected school sooner or later will catch on, and the administration may start an investigation. When the truth finally comes out, the president of the neglected school may write to the administration of the school where you just got tenured in the hope of exposing this egregious behavior. Is it illegal? – Not really, people manage to hold several part-time or sometimes even full-time jobs (not in the academia, though, this is why it's so quirky). Is it unethical? - Obviously, grossly. Especially if you are a role model (as we teachers are supposed to be) to the future employees that your students will one day become.
However, the school where you got tenured may not even care and you wouldn't even be punished. I guess from their perspective, it might be a sign that you are indeed entrepreneurial.
The school that you neglected, though, may introduce a specific moonlighting policy into its faculty handbook, which would make the new faculty scratch their heads and think, "Where does this come from?"