Though I am not completely sure why they would install two 500A shunts, the fact that they are in parallel probably answers the question.
When I use my Bow Thruster (which I sometimes refer to as the "Amel Battery Killer") in Aquarius we draw just over 400 amps. When I first bought Aquarius, I was not looking at the amp meter when docking, it was all hands on deck. I knew the Bow Thruster used a lot of current, but I also thought that the engine is running, and the 100 amp alternator is suppling a lot of the power... Wrong... When you are docking your engine is running at low RPM, and thus you will not get much current out of the alternator. O, and even if I had my GenSet running, my charger will only charge at 100 Amps MAX. So, I would still draw 300Amps, and that is still a massive current out of the batteries, especially lead acid batteries. During my daily routines I would try to keep the draw on our AGM batteries less than 30 amps... Didn't think much about the bow thruster, but it's one more reason to limit the amount of time you use the bow thruster.
Now to answer the question... I would think that Amel did this because at some point (Probably due to the bow thruster) there is the possibility that the current draw exceeds 500A. This would mean that the two 500Amp Shunts would have to be added to get the total amps in or out of the batteries.
Try looking at your amp meter while you use the Bow Thruster... In the A55 you might see up to 500amps. I see that the A55 is fitted with a 12 HP Bow "Thirsty" motor. That would be at the very least 380Amps! I would be really interested to hear your current draw when using your A55 bow thruster...
Please let me know.
Looking to put her on the hard soon in Krabi Thailand