Okay, you keep slightly tweaking what you’re saying. Saying you want the menu items on the children sites to be inherited from the main site is not the same as saying you want to ‘display the header.’
Are you trying to say: “I want the menu I use on my main site to be the menu for all my children sites.”
I’m trying here, man, but you’ve not given me a lot to go on. I know it makes sense to you, but you’re using non-standard terms, and I’m trying to convert them and figure out what you mean. You have to remember we’re coming from different ends. ?? It takes a while sometimes to figure out what everyone means, and text is a real shoddy way to do that.
There’s a thing called ‘Network Wide Menu’ – https://wpmututorials.com/plugins/networkwide-menu/
I hope that’s what you want. I honestly have no idea what you mean by ‘two headers.’