I might be and might not be here tomorrow.

[navi tasan]

1) I might be and might not be here tomorrow.
2) I might be and might be not here tomorrow.

Are both sentences grammatically correct?
Are both idiomatic

I think #1 is grammatical, but I suspect that #2 might be used in informal contexts.

 

[jutfrank]

You probably want this:

I may or may not be here tomorrow.

 

[Annabel Lee]

Or this:

I might be here tomorrow, and I might not.

 

[emsr2d2]

I'd use "I might or might not be here tomorrow".

 

[Skrej]

Only #1 is possible, but as you've seen, it's not the most idiomatic way to express it.

You typically see it as "I may/might or may not/might not X". I suspect usage is fairly evenly split between 'may' and 'might'. I certainly use both.