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So including relevant safety factors (extremely high ambient temp, high continuous current, highly uneven current sharing) 2 parallel groups of back to back FETs should be completely fine.
A note on board space - we can squish them together and add a heatsink on them to make the board smaller (for future revs once we are sure that it works and have tested thoroughly).
It would be a good idea to add thermistors near the FET on the board if testing proves that they heat up under high load. This was apparently discussed for MSXII: Power Distribution Revisions