I'm not sure I can see why you wouldnt go ahead and wire the two charger outputs to the two battery/banks. Eliminate the combiner, which is just another thing to go bad. I had one go bad on my BVI charter last year.
I would not try to combine the charger outputs.
Just because you had one go bad, of unknown brand, does not mean these are not very, very, very reliable devices. A lot of the charter companies do not use real ACR's or purpose built combiners they use basic relays. Because of this they can wind up with relay chatter when the banks combine. This can kill them in short order because they were not designed properly to begin with. A genuine Blue Sea or Yandina combiner will not chatter due to programming delay logic.
If the OP has a combiner he would be very wells served to keep it and just use the 10A output..
The failure rate of the Yandina combiners for example is well under .1% and they've sold well over 30,000 units, "
An annual failure rate of 0.0125% is why we can give unconditional warranty".
Not too many marine electrical products with that type of warranty or reliability. The Blue Seas ACR's and the Yandina combiners are very, very reliable and far more reliable than HEF or "Human Error Factor" when constantly manually selecting which banks to charge and then remembering to isolate them when the engine is off.
The OP can combine two outputs to make it a 10A charger.