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 two layers at an angle from the top to bottom all the way around to remove/peel the skin and expose the flesh. Make sure to trim off all the pithy white parts. Then, holding the grapefruit in one hand, release the wedges by using the knife to slice on either side of each wedge's membrane. Squeeze the remaining flesh and membranes of the grapefruit to release juice into liquid measure with juice from other grapefruit. So now you have one grapefruit in wedges and one completely juiced. To the grapefruit juice add the measured sugar and honey. Whisk to combine then taste. If needed add a teaspoon or more of brown sugar or honey to take away the tart of the grapefruit.2.
Marinate the chicken. Pour half of the sweetened juice into a plastic bag and add oil, salt and a few grinds of black pepper. Add chicken, seal and toss a bit before resting on the counter for 25-30 minutes.3.
Poach the chicken. In a large pot on low heat add the chicken, marinade and just enough water to cover the chicken by an inch. Simmer on low until the chicken is 155°F, checking with an internal thermometer. Remove from the pot to a plate and cover with aluminum foil, resting about 10 minutes to allow the carry over cooking to bring it the internal temperature to 165°F.4.
Cook the glaze. In a pan on medium heat add the grapefruit wedges and the remaining half of the sweetened juice. Bring to a simmer and cook until liquid is reduced and slightly thickened.5.
Grill the chicken. Heat grill or grill pan to medium high and brush lightly with oil. Remove chicken from plate and pat dry (if poaching the day before, uncover the chicken for an hour inside the refrigerator to dry out the skin, then allow chicken to come to room temperature before grilling). Place skin/smooth side down on the grill and cook 2-3 minutes, glazing immediately then flip and cook 2 minutes more, glazing the skin/smooth side. Remove to a serving plate and pour remaining grapefruit sauce over the top.