I will assume you have a regular German job, where you are employed by a company. That means you are covered by normal German labor law.
Obviously, if you work for both companies at the same exact time, that is fraud. Your contract is not based on your results, but on time. You are contracted (most likely) for 40h. So if you tell the German company you work for them Monday to Friday 09:00-17:30 and the Dutch company you work for them Monday to Friday from 10:00-18:00, then that is fraud. Plain and simple. It's illegal.
So lets assume you manage to actually cram those hours into a day without them overlapping.
You are not allowed to work more than 48h a week as an employee in Germany.
It does not matter if that is a single job, or a combination of multiple jobs:
Arbeitszeiten bei mehreren Arbeitgebern sind zusammenzurechnen.
You are also not allowed to work more than 8 hours a day, meaning if you want to do the maximum 48h regularly, you need to stretch that over 6 days.
In addition, there is a resting period of 11 hours that you need to adhere to. If you start working at 8:00 in the morning, you have to stop working at 21:00 to be able to be eligable for work next day at 8:00, no matter what. The maths of craming two non-overlapping 8 hour working days in the 13 hours that is the day after the resting period just does not add up.
While you can work more hours or irregular hours (and many people do) that can only be temporary.
If you do sign a work contract that would bring your normal hours to over the allowed legal maximum, that contract is void:
Führt der Abschluss eines zweiten Arbeitsvertrags mit einem anderen Arbeitgeber dazu, dass der Arbeitnehmer nach § 2 Abs. 1 S. 1 2. HS ArbZG die regelmäßige wöchentliche Arbeitszeit von 48 Stunden überschreitet, hat dies grundsätzlich die Nichtigkeit des zuletzt abgeschlossenen Arbeitsvertrags zur Folge.
LAG Nürnberg, Urteil vom 19. Mai 2020, Az. 7 Sa 11/19
So as a regular employee under German labor law, what you want is not possible. Doing it anyway is somewhere between actual fraud and breach of contract, neither very favorable to your immigration status.
That said, you do not need to be an employee. If you become your own boss, through either being an independent contractor, or company owner, you can (mis)treat yourself as you choose. If, in addition, you sell results instead of time, you do not have the "fraud" problem where you promised two entities that you worked for them at the same time. If you make widgets, you can absolutely make two widgets on Monday and sell one to company A and one to company B. However, that comes with almost zero protection by the law and all the risk on your own shoulders.
For the independent contractor/company owner route, you will need a tax accountant and maybe an immigration attorney for the details. That is too much paperwork to just go forward with in hopes of good luck and forgiving authorities.