Say I need 600 Amp Jumper leads to start a diesel, can I use 2 sets of 400 Amp leads between the 2 batteries? That is 2 positive clamps on the positive posts and 2 negative clamps on the negative posts. Would they share the load?
Theoretically they would share the load assuming an even contact on the clamps. Since metals in generally have an positive temperature coefficient of resistance the leads automatically distribute the current equally.
My gut feeling tells me that a significant part of the resistance "happens" on the clamps, so it would be difficult to balance the load (every clamp contact in a pair of clamps needs to have the exact resistance). In practice I would stick with one set of 400A leads and be confident that it would work, but everyone needs to decide for himself if he will take the risk (Wear gloves in case something does not work as expected).
I once started an 3.5L diesel with a set of 16mm² copper leads, the leads got warm but the car started.
According to the science YES they will equally share the load, as long as the conditions between the two cables are the same. ensure they're both connected the same, both have the same length of cable, and are both the same tolerance.
According to the rules of catching things on fire, make sure the above precautions are met. Other than that. Two wires in parrallel will share a load
EDIT: DO NOT use just one set of 400A leads as the vehicle uses the most energy at startup, and jumping is when most of the cranking occurs, and thus most of all the current will be active during startup