dc - dc ctek D250s Dual

Try ringing this company tomorrow Cteck

They think are either the state distributor or national distributor or something like that for Ctek. I have called them a couple of times with technical questions and they were very helpful.

Two together will not work.
You need to separate the chargers so that only one is in each circuit.
With two chargers in the same circuit, the voltage output of the first charger will drop below the 13.1v minimum input voltage required once the battery connected to it has neared a full charge and therefore the second charger would not receive the minimum voltage of 13.1v to open the built-in isolator.
I installed a D250S in my camper van to provide a multi-stage charging process to the two 100Ah batteries in the van.
I subsequently added another D250S to my auxiliary battery in the vehicle to provide a better charging process to this battery. I wasn't happy with the VSR previously controlling this circuit. The battery was not receiving a 100% charge rate.
The way I have wired it up is to have one cable running from the starting battery to the tub area of my dual cab and terminating with an Anderson connector.
From this I run two circuits.
One to the D250S connected to the auxiliary battery in the tub.
A second circuit by way of a "patch lead" runs out the back of my tailgate and connects to the camper van's input cable. The other end terminates at the second D250S in the van.
In essence, I have two "separate" circuits, each with its own dc-dc charger to maintain the charging process of the battery bank connected to each and fed from the primary starting battery which in turn is isolated from each remote battery bank by the dc chargers.
Each charger receives its input from the starting battery, once a voltage exceeding 13.1v is detected and provides a multi-stage charging process to the batteries connected to it.

Another option is to use a Smartpass in conjunction with a dc charger but because you already have two D250S chargers, just ensure each is separated from each other and all will be well.

Depending on the age of your unit it will either contain blade or soldered fuses. The unit is factory sealed so if it is under warranty you might want to send it back. Otherwise drill out the seal and undo the screws. Test the input/output with a multimeter before committing to surgery though.
I run two dc chargers. The feed to the camper runs direct off the vehicle's deep cycle battery and the chargers live together perfectly happily.

The D250S is a charger, not a power supply, so the one in the car will not run the one in the trailer.

If I read Sand Man's post correctly, I have a system like him. Two circuits, one to power the system in the canopy and a separate one to power the trailer system via an Anderson on the towbar.

Schematic attached for info.

It looks complicated but it's not.

The aux battery is in the canopy in my ute.

The red and black busses are heavy so I can winch off both batteries or start off the aux if required.

When the crank battery comes up after engine start the Redarc VS12 module activates the 300A solenoid. This effectively switches on the heavy duty bus back to the canopy. It also switches on a feed to the Anderson on the towbar, which in turn powers a dc-dc charger in the trailer. That Anderson is not live when the engine is off and the vehicle unattended - something I prefer.

Back in the canopy the Ranox dc-dc charger gets powered up and looks after the aux battery.

When the bus is live (engine running) the relay in the canopy powers the fridge and a spare socket from the alternator system. When the engine is off the relay powers the fridge from the aux battery. I did it that way because I prefer the Ranox to be charging an unloaded battery (ie, not being influenced by an intermittent load as the fridge cycles. Not 100% necessary, but I did it because I could.)

The other stuff is self-explanatory - solar reg, a couple of cigar and Merit power outlets and an Anderson power outlet.

Cheers

Reads like you have the D250 trying to supply charge to the input of the DC to DC device. The charging output of the D250 will be looking for a rising terminal voltage of the battery it thinks it is charging. The set up is not going to work. I suggest you take one or the other out of the line and see what happens. I am quite sure you will get positive results.

Iza

Ignoring the fact as to if it will work or not why would you want to?

The first charger can supply a maximum of 20A to the in car aux, if the batteries in the trailer are down the trailer charger will try to pull 20A + to charge the trailer batteries, ie you will get no charge to the in car battery till the current draw from the trailers DCDC drops below 20A.

Very inefficient setup, about the only advantage of doing it this way is that you will limit the draw from the cars alternator to around 30A or less if there is a need to.