I'll focus on the formal/legal aspect of your question, as the issue of practical feasibility has been covered in other answers (in this and the linked-to threads). A second full time employment will most likely be impossible for formal/legal reasons alone.
In many places in europe a "PhD position" means you are an employee of the university. Sometimes these are part-time and sometimes full-time positions. If full-time, you have a contractual commitment to work 36-40 hours per week for your university, so this would absolutely preclude another job. If part-time, the contract would most likely require you to seek assent from your superiors, e.g. HR but de facto your supervisor, for any other employment you take on. The expectation with part-time "PhD positions" is that you work on your PhD project in your spare time and on your employment duties (teaching, research, administration) during your work hours, even if this distinction is often fictitious. You won't be able to do this if your spare time is consumed by another job. Thus you probably will neither receive assent for taking on a second position with any significant work hours nor have sufficient time for your PhD project anyway.
There is one exception, namely that the second job directly contributes to your PhD project. For example, industry internships in an applied/engineering filed, or government internships that allow you to conduct participant observation in political science are not uncommon. Nothing in your question suggests that this exception applies.
Moreover, the contract for your prospective second job will most likely also preclude you from taking on another full-time position.
You will have to talk to your supervisor about this, but it sounds very unlikely that you will be permitted and/or able to work full-time while being on a PhD position. If you are dead set on the second job, you could try to negotiate a later starting date of your PhD position and consider already enrolling as (self-funded) PhD student before. But even then, your chances of success are small, because your supervisor probably wants to fill the position soon.