ArchiCAD can currently only open one plan (Story) window at a time, and can only display one layer combination across all the open model windows. There have been a lot of requests to change this situation and surely the future will bring some improvements.
But that doesn't mean you cannot use existing ArchiCAD features to do almost what you want. Since you're only looking to change the zoom level and since you want to work in both places (as opposed to working in one and just looking at the other) all your options involve manipulating the single floor plan window:
1. Save a view of each in your Working Views folder.
2. Use the Previous & Next view buttons/commands (assign shortcuts if you haven't) to simply toggle between the two zooms.
3. Use the often overlooked Home View along with Previous/Next, 100%, or Zoom Extents.
Hope that helps.
Regards,
Geoff Briggs
I & I Design, Seattle, USA
AC7-28, M1 Mac, OS 15.x
Graphisoft Insider's Panel, Beta Tester