Remove giblets and necks; save for another use. Rinse chickens. With poultry shears or a knife, split chickens lengthwise through breastbones. Pull birds open and place, skin up, on a flat surface. Press down firmly, cracking bones, until birds lie flat.
Insert a metal skewer (you need 4, each 10 to 12 in.), across 1 chicken through drumstick (knee bent), then thigh, over backbone, through opposite thigh and its drumstick (knee bent). Repeat step with remaining chicken. Then, on both chickens, push another skewer parallel to the first through skin of outstretched wing into breast, over the backbone, through breast, and out through skin of opposite wing.
In a blender or food processor, whirl garlic, onions, ginger, coriander, pepper, and fish sauce until a coarse paste. Rub paste all over chickens.
Prepare barbecue for indirect heat. If using charcoal briquets, mound and ignite 60 briquets on the firegrate of a barbecue with a lid. When briquets are dotted with gray ash, in 15 to 20 minutes, push equal amounts to opposite sides of firegrate. Add 5 more briquets to each mound of coals now and every 30 minutes while cooking. Set a drip pan on firegrate between coals. If using a gas barbecue, turn all burners to high and close lid for 10 minutes. Adjust burners for indirect cooking (no heat down center) and keep on high.
Set barbecue grill in place.
Lay chickens, skin up, on grill. Cook until meat at thighbone is no longer pink (cut to test), about 1 1/4 hours.
Pull skewers from chickens, cut chickens into pieces, and serve with Thai chili sauce.