If you leave a gap or put in a block or something, your eye will be 'drawn' to that point. Then, the eye will see the 'difference', and perhaps, you'll get the illusion that there are still 2 different rooms.
If you want the 2 rooms to look like 1 room, then obviously, you don't want anything up there AND you want the SAME molding to boot. Anything else, and you'll lose that desired effect.
Now, if you DON'T care what look-and-feel you get, then I'd consider some sort of Victorian Molding to 'separate' the 2 types of molding. You could construct something out of various types of molding, or you can buy something out of a catalogue.
As an aside, a few years ago, I refinished a 1st floor powder room. Running across the middle of the ceiling was a kitchen stove duct. (The powder room was between the stove and the outside wall so the only way to vent it was to 'box' the duct onto the ceiling of the powder room.) With that, my wife wanted to 'border' the ceiling with wallpaper. She asked, "Should I put the border on the UNDERSIDE of the drywalled-vent, or should I 'stop' at the side and just continue it on around at the other side of the 'box'?" I said if you put anything on the 'box', your eye will see that the wallpaper is 'upside down'. So, she opted to create an 'illusion' that the wallpaper just continued 'through' the box. WHew!!! I guess all I'm saying is that if the eye is going to be drawn up there, you'll need to 'mitigate' the difference between the moldings by putting something up there. BUT, of course, you will inject the feel of 'separating' the rooms.
Those are my thoughts. It depends on what 'feel' you want. If it's one room, I'd change the molding. If you don't care for that, then it's a crap shoot where you try to convey some sort of illusion. My best to ya and hope this helps.