Complex web design considerations - tips appreciated

I'm working on a Web remake of an existing WPF application, it has a pretty complex screen structure. For instance, screen A may call screen B which may call screen A again, possibly even for the same data, but with a different context.

There may also be instances where two screens should be readable at the same time, because the user may need data from one screen, to enter data into the other.

I refuse to use popups for this, they're a nightmare for the user but also to keep track of inner dependencies technically, but, considering the complexity of the screen structure I would appreciate any tips.