from IWP guide:
New Window (script step)
Re: IWP - New Window/TabDoes not open a new physical browser window, but instead opens a new virtual window within the existing browser window. Although each virtual window can display a different layout, table, found set, or sort order as is possible in FileMaker Pro windows, web users can only view one virtual window at a time.
IWP is basically in a frameset and you cannot have multiple "windows" open, so that you can view them together at the same time. You can have navigation to switch the "frontmost" window, however. Use the Select Window script step, for example.
If there are ways to use portals to view different "tables", you can try that.