I'm post-doctoral reearcher in two countries. I have double affiliation and I receive salary from two countries, whereas my total occupation rate (FTE) is limited to 100% of the legal working time. This is named a cross-border commuter situation.

In such a situation, the administrative work becomes large. But the most difficult is that: it becomes difficult to get real help.
- Human Ressources have other people to manage and in general don't treat such cases (too complicated, too sparse).
- Friends and colleagues usually advise you to hire a professional to manage this.
- Professionals are rather meant to help employees from the private sector (or freelancers), and this double taxation situations became somehow a juicy business.
- On the other side, the European networks of researchers (EURAXESS) were not willing to help me in that case [could have been an isolated case]. 

Therefore, I understood this year that scientific mobility is not exactly ready-to-go issue, unless it would be for a definite time, within organized grants such as Marie Curie Fellowships, where namely you keep being paid by ONE institution, wherever you go. But then, how to proceed when a Marie Curie Fellowship is over, and that you need to work further with your network? This is where double employment can happen.

I therefore decided to document my adventures with double taxation in cross border situation, in view of helping other scientists like me, who may get into troubles because of their ignorance regarding the laws and the entry and departure procedures from a country.

Because, let's open the eyes, who is really leaving its country? Scientists do, and surely many people (refugees, surely!). There as scientists we are a problematic group of people, as we are looking forward to push boundaries of knowledge. But for this, we also need to push some administrative boundaries. Who can understand that? Few people. Hence, it deserves to be written how to do it.

Revenue taxes

https://europa.eu/youreurope/citizens/work/taxes/double-taxation/index_en.htm

1. Check the treaties that have been signed by both countries before leaving to your next destination! They are available in both languages of each country where you work(ed).
https://ec.europa.eu/taxation_customs/individuals/personal-taxation/treaties-avoidance-double-taxation-concluded-member-states_en

If you are in my case, you don't speak any language of the countries you are working in, you have to ask friends, or to your most kind employer. Thanks to technology, you can also use the Mobile application of Google Translate that can actually perform character recognition, and help you in understanding the juridic text... more or less.

2. Declare your presence officially in a VERY short time. If not declaring yourself (urgently) to the Mainhall in Germany, you may get maximum tax rate, which may be higher than your actual salary (?). My experience was the following. In Germany, you have 7 days to register after your contract starting date. If you don't do after this period, your tax level will increased to Level 6, which may remove 500 € / month from your salary. This is actually retroactive. In my case, the money was taken 3 months after the starting date, at once, without notification. 1500 € gone on a salary of 2000 €. Nice surprise when you just invested all your savings in moving.

3. Hire a tax advisor to help you filling the tax declarations in both countries. A mistake can cost more than your savings.

Health insurance

This part is very complicated.

https://europa.eu/youreurope/citizens/health/when-living-abroad/health-insurance-cover/index_en.htm

1. You don't have the right to pay double health insurance (i.e. in two countries). Hence you should ask for the right forms (S1? A1?) to the health insurance that you would like to apply to. For this, call your health insurance company, and ask them to provide one of these forms.

2. When arriving into the new country, you MUST inform your former health insurance of your change of situation. If not done, they can require you thousands of Euros even, 5 years later. Think that human ressources may not inform you of these necessities.

If you fail to do these steps, don't panic too much (still a bit). It exists EU forms aimed at solving these kinds of problems. However, they are difficult to obtain in the given time.

My experience is the following. I forgot to unregister from the townhall of Berlin. At that time, I  received kind letters from health insurance that were asking about my new condition. Given the high stress level, the permanent rush and the required scientific mobility (I changed flat 5 times last 4 years), I forgot to open these letters sleeping in my boxes since 2014.

As a result, I got a request in 2018 from the former health insurance company to pay rights for about 1 year: ~ 4600 € within a delay of 3 weeks. The most striking thing was: they were NOT indicating HOW to solve this problem. Warning: Don't pay the amount without being totally sure that you should. Otherwise you may put yourself in an even more illegal situation by paying health insurance twice. 

3. In such a situation, stress level increases drastically and ability to work efficiently decreases violently. You may need to ask help to many people at once. Here is what actually worked the best for me.

3.1. Call / contact the health insurance who is threatening you. Ask them for explanations. In my case, they mentioned that EU form E104 was necessary to justify that I already paid my rights in another country.

3.2. Call the health insurance that can justify that you have already paid. Sometimes, they produce the form S041 instead of the E104. It is okay', it is the same form, just a recent revision.

3.3. In my case, the German insurance had refused the form provided by the Czech health insurance.

After going by myself to the German health insurance, I figured out that the problem was NOT the form number, as they were seemingly claiming by email, but only that the writings on the form S041 were ... too small. They did not see the main information in this form. I went there and showed them the right box, then they understood, and solved everything quickly.

3.4. In case of more serious complications, EU institution exists to help you! They are named SOLVIT. Here they are. http://ec.europa.eu/solvit/

However, they can help only if you did not contact a lawyer to prosecute one of the companies. So think twice before following advises of your friends telling "hire a lawyer!".

Social security

In which country should I pay my social security rights?

Procedure is still under investigation. So far, the E104/S041 form generated by previous health insurance should be sent to the Social Security institution, so that they would decide in which country you should pay the rights. They may emit a form A1 or S1 containing this decision. This form may be sent to the 2 Social Security institutions so that they would bridge themselves to redistribute your payment. Maybe it should be also provided to your health insurances.

Retirement plan

To be developed, if retirement still exist in future.