I was wondering what the effect of a vehicle with two 12 volts batteries in parallel would be on the test results? This is a common scenario in North American diesel pickup trucks.
It is getting more common now to see 2 x 12 Volt batteries used in Parallel especially in Commercial Vehicles due to the High Currents Drawn through the system and the need for a Larger Capacity to Operate More Accessories.
In regards to testing these set ups it is possible through Pico Diagnostics but you would need to remember that as its a Parallel Connection you would need to add the CCA of both batteries together in order to run the test.
The only real problem by doing this a Good battery could mask a Poor battery by giving an overall good capacity result therefore we would recommend that you test the batteries individually to get a more accurate result for each battery in the vehicle. Also you have to remember that as the batteries are interlinked the cables could also cause a difference in the overall result due to either cable condition and/or connection condition so again we would recommend that you test each battery individually.