Working in 2 EU countries simlutaneously doable?

[AlexInGermany]

Is it possible to work in 2 EU countries simultaneously ?

AI is saying there is some system which flags people who contribute social insurances to more then 1 EU country as non compliant.
Tax is also getting complicated but seems doable, but I see no solution for social insurances.

I was thinking is there is a way? maybe people or tax/insurance advisers from border cities (grenzegenger?) know how to do it?

I found something like A1 Certificate but this also seems only doable if company requests , no from personal end, and only for temporary duration.