I would definitely go with two separate displays if you have room for them.
The Zeus has so many useful screens that you will regret having only one of them. The 8" Zeus screen is SVGA resolution and for doesn't really work as a split screen.
One flaw (IMHO) of the Zeus is that you can't invoke a list of AIS targets the plotter screen; only on the radar screen. If you're using separate AIS, then maybe this is not really important to you (you do get the AIS targets themselves on the plotter screen, and MARPA targets, too, just not the list), but I for one would really love to be able to have radar with AIS target list on one screen, and plotter, possibly split with the SailSteer screen, on the other. That would really be cool.
If you don't need the AIS target list, then it is true that the radar overlay is very good (IF you have a good heading sensor). On an offshore passage
, I set up a radar guard zone, then switch back to the plotter screen with radar overlay. This works ok, but sometimes in dense ship traffic you would really like to see both the radar and plotter screens at once.