I think i do not understand your issue properly.
Can you explain a little bit more what you are trying to do (without using the word flag 
) I do not understand what you mean, when you say that you want to set flags between 2-pages.
Please describe in other words. My english is not perfect as you can see .
I just see you are trying to work with the Wix-Storage {session}.
So i assume, you want let communicate two pages, right?
Describe what should happen, step by step.