Oh i'd tried Login already a few ago. Basically my Login Solution is using the same core processor and - well - its kinda the same at all - the only difference is - i need some more logic and custom events after the basic authentication, the hook mechanic was just not enough for that.
Just to be sure i just tried it once more with the newest version of login - its the same behavior. I can login on the current context - but i can't spread the login session to other contexts.
For the current project purposes i will stick with multi contexts, cause just Staff Member will need to login twice per session i guess... maybe i'm able to find a fix one day (i still dont wanna accept - that it does'nt work).
The funny thing is - over my Research i figured - that's an issue which is arround since a few years - and there was already a few fixes - which worked temporaly - but they stopped working on later Releases.
Anyway - i learned again a lot about the modx core through this

- thanks for your input and patience