Easy just think of a good ol 2 or 3 cell flashlight. The cells are lined up "in series." So you hook one charger NEG to the NEG of the target battery, and normally you'd hook the POS clip of the charger to the POS of the battery. Instead, you hook that POS clip of the first charger to the NEG clip of the SECOND charger, and hook the second charger POS clip to the POS clip of the target battery.
If you have one charger and one good battery to boost the target, you hook the "good battery" in as if it was the second charger.
I've done all sorts of idiotic redneck crap at times. One below zero cold day, and I had nothing else, I boosted one vehicle to get it running with a DC welder set fairly low. This was "back before" computerized engine controls!!!!
