Just put in an isolation valve between the 2 tanks.
My N46 had about 5 isolation valves for the 5 tanks.
I think they used the FW as ballast and buried them in the keel.
First cruise, I did not top off the water tanks and ran out of water. I did not discover the 5th isolation valve till the end of the cruise. After that, I topped off all the tanks isolated all but 2 tanks, fwd port and starboard tanks. Never had trouble again. The 5th tank was the smallest tank.
I did install a 150gpd water maker...... The intent was, if I started to get low, turn it on and over 5 or 6 days, fill the water tanks. Never used it.
The meek will inherit the earth but, the brave will inherit the seas.