Hey thanks again for the answer,
unfortunately i am not at my pc right now.
I redid some things now and it seems that it could be working that way too.
Each level i have will create a User Widget Creation for a player loading that level.
GameInstance Init -> will prepare the default when the game starts for each player.
As far as i know, every Player will have an own GameInstance.
When i play around with delays i almost get the right result, but im not sure about that workflow of delays in order to handle each gameinstance for each player.
The best result was that both Player could receive Input, but for the client that input for UI Buttons was only 1-2 seconds enabled and then gone.
However to refer to your answer, i did the creation in GameInstance as well, and i think using the enable Mouseclick for touch input should work for all testclients.
When simulating and playing with delays of different sizes and different locations i can observe the gameobject browser, that sometimes both (for 2 Player) GameInstances were created and both PlayerController were created, sometimes the load StartMenu Map only works for one player and the second stays inside About_Blank for some reason.