Combine the orange juice, lemon juice, sugar, salt, and orange slices in a medium nonreactive saucepan over low heat and bring to a slow simmer. Cook for 6 to 7 minutes, until the peels are tender and the centers of the orange slices are starting to become tender and translucent but are not falling apart. If the peels aren’t yet tender enough to cut with a fork, keep simmering until they are. Using a slotted spoon, carefully transfer the orange slices to a plate. Continue to simmer the syrup until it has reduced to 1/2 cup, anywhere from 5 minutes to 15 minutes, depending on how long you simmered the orange slices and the size of your pan.