Right, I think I have it.
TOUCHDOWN PROTECTION (wheels 2 & 4).
Prohibits wheel braking when the A/C in the air.
TOUCHDOWN/HYDROPLANE PROTECTION (wheels 1 & 3).
Senses a difference between Groundspeed (from the ADIRU) and wheel speed (from the anti-skid transducer) and releases the wheel when the speed difference is greater than 50kts.
Therefor all 4 wheels have touchdown protection, and wheels 1 and 3 only have hydroplane protection. (the touchdown protection being sensed in 2 different ways)
LOCKED WHEEL PROTECTION-
Compares the speeds of both inboard and both outboard wheels (i.e, 1 with 4 and 2 with 3) and releases the slower wheel when there is a 30% speed difference.
So i suppose the reason for the pairing is that locked wheel protection would be the most likely one to be activated, so this one operates symmetrically?