In a blender, blend the sugar, fish sauce, soy sauce, oil, lemongrass, coriander and garlic with 1 tsp salt until smooth. Divide the mixture between 2 large bowls. Add 1 chicken to each bowl with ½ tsp freshly ground black pepper. Turn to coat the chickens inside and out. Cover and refrigerate for at least 12 hours or up to 1 day.
Prepare a covered barbecue for high cooking over indirect heat.
Pour off half of the contents (190ml) of each beer can. Place the cans on a large, heavy rimmed baking tray. Wipe excess marinade from the chickens and carefully position the chickens over beer cans so they stand upright (the legs should rest on the tray to help steady them). Carefully place the tray over the unlit portion of barbecue. Cover barbecue.
Cook chickens, rotating 180 degrees halfway through cooking, for 1¼ hours or until the chicken skin is golden brown and crisp, juices run clear and a meat thermometer inserted into the thickest part of the thigh registers 73°C. (Alternatively, roast the chickens in an oven at 200°C or 180°C fan-forced for 1¼ hours or until cooked through.)
Using tongs, carefully transfer chickens and cans to a large carving board. Carefully lift chickens from cans. Place chickens on the board and set aside for 10 mins to rest. Discard cans.
To make the sweet chilli sauce, whisk the sugar, lime juice, vinegar and fish sauce in a medium bowl until sugar dissolves. Stir in the garlic, chilli and coriander with ½ tsp sea salt flakes and ½ tsp freshly ground black pepper. Drizzle a small amount of the sauce over the chickens. Serve with the remaining sweet chilli sauce.