I have duel batteries with 3 on/off switches. They are a port/stbd and parallel switch.

When I load test both batteries, the batteries test good... Question?? If I disconnect the 2/0 battery at the Port on/off switch, can I load test the battery through the cable? I wish to check the cable to see if it's good. Can I load test through the switch?

If you are wanting to test the cable to see if it's good, why not just test the cable? Disconnect both ends and check resistance with an ohmmeter.

That's the first thing I did. It measured good. So I want to put 1/2 the CCA through the wire. It should measure good with a Carbon Pile Load Tester if the wire is good, YES? NO?

Load tester removes surface charge from the battery. Same as leaving a car's headlight on for a certain amt of time (I like 10 minutes) and checking battery voltage after.

don't get what load testing thru cables would do,but I don't get much.

I do it every day , Gives you and indication of complete circuit condition.
You would be surprised how much resistance you will find doing that.
After your load test , feel the cables and connections for heat , you found your resistance.
You wont believe how many Junk battery switches I find that will only take a 100 amp draw.
Testing individual components first is fine , but I like to test the whole circuit first , then go back and find the Culprit , usually more than one problem!
Ive done it that way on everthing from Airplanes , autos , constr equip , farm equip , you name it.

When I load test the Batteries, they test sat. If I load test through the cable, it fails the test. I have taken a resistance reading of the cable and that checks good.

Now with 4 foot of 2/0 cable and the battery showing 12.5 volts the results should be, IMO, pulling 475 amps should be when load testing,
Voltage drop: 0.30
Voltage drop percentage: 2.37%
Voltage at the end: 12.2

So I'm thinking that there is a problem the ohm test is not picking up and I have a cable problem...

Load testing a battery does much more than that.....

A large cable wont show much resistance on an Ohm Meter even if its bad .
Now start eliminating components and cables , you could have corrosion in the cables were the lugs are crimped .
Ive had cables rotted apart inside and look perfect , You can also do the stretch test! put one end of the cable in
a vice , put a screwdriver through the lug on the free end , and pull like hell. If the cable is bad it will stretch at the point of failure , or the
ends will come off. Any good crimp on a 2/0 cable should hold more than one man can pull. Sounds funny but it works.
Best bet is replace all the bad circuit cables and components that test bad , Make Sure your Grounds
are clean and Tight. Ive also seen the lead on battery post oxidize so bad that they have no connection.
you have to use a wire brush to clean all that **** .

Thanks for your response. All terminals are clean. I might have to remove cables, but seem tight