I am not sure where you are based, but in Germany (and I assume in many other countries) you have to disclose and other parallel employment in detail to the respective other employer for tax reasons. Not to disclose this a second job is thus not only "frowned upon" or unethical, but illegal.

There are also laws in place how many hours you are allowed to work per week (these laws are in place to prevent people from being overworked) and having two fulltime jobs (around 8 hours/day) definitely exceeds that limit (I believe it is 48h/week) by a lot. So another reason this would be illegal.

On top, teaching does not only entail giving lectures, but preparing them, giving exams and correcting them, in some cases being a (co-)supervisor of bachelor or master theses etc. While it might be somehow possible to hold all lectures online for two full-time positions (but that only works out if there is absolutely no overlap), I highly doubt that it would be possible to satifactory fit all those other duties for two full positions into the time one person has. I believe that if either uni were to know about the fact that you took on another full time position with another employer, they would not be happy about it and seriously thinking about terminating your contract.

I am not sure where you are based, but in Germany (and I assume in many other countries) you have to disclose any other parallel employment in detail to the respective other employer for tax reasons. Not to disclose this second job is thus not only "frowned upon" or unethical, but illegal.

There are also laws in place how many hours you are allowed to work per week (these laws are in place to prevent people from being overworked) and having two fulltime jobs (around 8 hours/day) definitely exceeds that limit (I believe it is 48h/week) by a lot. So another reason this would be illegal.

On top, teaching does not only entail giving lectures, but preparing them, giving exams and correcting them, in some cases being a (co-)supervisor of bachelor or master theses etc. While it might be somehow possible to hold all lectures online for two full-time positions (but that only works out if there is absolutely no overlap), I highly doubt that it would be possible to satifactory fit all those other duties for two full positions into the time one person has. I believe that if either uni were to know about the fact that you took on another full time position with another employer, they would not be happy about it and seriously thinking about terminating your contract.