Water tanks on the 43

I fill both tanks from one side, ususally from the port side. I open both fill ports, and when full water flows out of the fill pipe on both tanks. The is a one way check valve under the cockpit floor but I haven't looked into how it is plumbed closely.

I have always had to fill on each side.
There is a check valve between the tanks on mine.
1980 -46.
My plumbing is forward of the aft fuel tank too, I can get at it from the engine area.
Looks like yours is aft of the tank. Looks much more difficult to access.

I'm pretty sure the check valve is to keep the house side water from flowing into the tanks if on dock water.

Can get to each tank from the fish box openings after the liner is pulled.

You may be right about that.
I have two check valves in that circuit that I know of.
One is between the house water feed and the dock water feed at the pressure regulator and the second is at the Y connection between the two tanks so what you say makes sense.
The hose between my tanks is 1/2 dia. then reduced at the tank itself to 3/8 dia.
Each tank has it's own fill.

On spring commissioning this year I happened to keep an eye on them as they filled.
Although water may run between the tanks, it is so slow that I really need to use both fills.
The Starboard tank, for instance, will be overflowing out the fill long before the port tank looks like it even has any water in it.

Also, mine are bladder tanks, not the original poly tanks.

The cross feed line on mine is 1-1/4" (?) PVC. Much larger than yours which is probably why the both fill at the same time.

Interesting.
Mine are all hose hose like Shields brand. No PVC.
Hose up to the regulator, then copper tubing with flare fittings throughout the rest of the boat.
Copper from the dockside valve to the regulator also .

I did an experiment filling the water tanks today. I can fill both tanks from one side if I fill them slowly. If I fill with the hose on full, which ever side I fill will overflow first. I can watch the gauge balancing the tanks (gauge goeing up if fill stbd first etc). At full hose flow both tanks will accept toping off after they balance themselves.

More on the tank filling mystery resolved. The tanks will not begin to cross fill until the one being fed reaches at least half full. That is when the water can cross flow throough the T shown. The check valve below the T, once primed, prevents water from draining back to the tank(s) through the hoses running to each tank bottom when the tank level drops below half. If/when the check valve dies (as mine just did) the water pump will not prime or stay primed.

I have no idea why they didn't just connect the hoses running from the bottom of each tank to a T at the level of the tank bottoms instead of going the check valve route.

91-46. Mine is a little different. I have one fill cap in the cockpit For both tanks. No check valves in the fillside just the T to both tanks and no check valve in the drain sides at the bottom. Both tanks fill until until port tank is full then starboard tank tops off, then overflow comes out the port vent in the cockpit. Everything worked great until the gasket on the sensor in the port tank started to disintegrate and leak water out the top of the tank and into the bilge. Had to pull the sensor and replace the gasket and the assembly. Very very difficult. It is on the forward side of the port tank towards the engine room. It is only accessible in the dark and with one hand. 5 screws not spaced symmetrically plus the gasket holes and the holes in the top of the tank all need to be lined up of course. It took me lots of hours to reach my arm in up to my shoulder while curled up in the engine room strainer area. But it works great again. overflows to the cockpit when both tanks are filled.