On a work surface, press down on the chickens so they lie flat.
In a small bowl, combine the oil with the garlic, lemon zest and a generous pinch of salt and pepper.
Rub the oil all over the chickens.
Grill the chickens over very low heat, turning occasionally and brushing with the lemon juice, until lightly charred and an instant-read thermometer inserted near the thigh joint registers 165, about 40 minutes.