The way the sewer lines are set up in our '64 GT you can do this, with mixed results.
The sewer line is a "Y" shape with the grey water normally by-passing the black tank completely and we don't have a grey tank. To store grey in the black tank, you would put the "termination cap" on the exit portion of the "Y" and open the valve to the black tank. The grey will back up to the black tank. This works on our model because the black tank is below the floor, some models are not like the '65 Caravel
amog others, which are above the floor so it would back-up to the shower (yuck!).
The only problem with this whole thing is when you go to empty the tank. You close the valve to capture the contents, but there is a small amount trapped between the term cap and the valve. So when you remove the term cap, some
spills out (double yuck!!) Yes, it's diluted...but I still don't want any on me or at my campsite!
You may want to look into a blue belly to handle the extra grey in between black dumps. The blue belly can be emptied of the grey into the restroom drain...or saved until you go to the dump station, effectively enlarging your capacity.
If there were a way to capture your grey overflow and fill it to the top of the black tank, rather than from the bottom in my scenario, the concept would work a lot better.