Prepare the grapefruit. Juice one grapefruit into a liquid measure. On the second grapefruit, slice the bottom and top to level it, then starting at the top where the white pith ends and the pink flesh begins, cut between the 2 layers at an angle from the top to bottom all the way around to remove the skin and expose the flesh, making sure to slice off all the pithy white parts. Holding the grapefruit in one hand, release the wedges by using the knife to slice on either side of each segment.
Place these wedges in a medium saute pan. Squeeze the remaining flesh and membranes of the grapefruit to release any juice into a bowl. Add the sugar and honey. Whisk to combine, then taste. If needed, add a teaspoon or more brown sugar to take away the tart of the grapefruit. Pour half of the mixture into a saute pan with the grapefruit wedges. Bring to a simmer and cook until liquid is reduced and slightly thickened, about 8 minutes.
To the remaining half add the oil, 1 teaspoon salt, and a few grinds of black pepper. Whisk to combine and pour into a plastic bag. Add the chicken, seal and toss a bit before resting on the counter for 25 to 30 minutes. Cook's Note: Any longer and the acid in the grapefruit juice will "cook" the chicken in the marinade and ruin it.
Heat a grill pan to medium high and brush lightly with oil. Remove the chicken from the plastic bag and shake off the excess marinade. Place the smooth-side down on the grill and cook 2 to 3 minutes, then flip and cook 2 minutes more. Remove to a serving plate and pour the grapefruit sauce over the top.