To make the marinade: Combine the orange juice, vinegar, ginger, and orange zest in a small bowl.
Put the pork tenderloins in a shallow pan and pour the marinade over them. Refrigerate for 4 to 6 hours.
To make the Orange-Ginger Sauce: Melt the marmalade in a small pan. Add the ginger, soy sauce, cider vinegar, and mustard, stirring to combine; keep warm.
Prepare the grill with a medium-hot fire.To gauge the heat, hold your hand palm down about 4 inches above the grill. If you can hold it there for only 5-6 seconds it means you have a medium hot fire.
Remove the meat from the marinade and season it with salt and pepper. Grill, turning at least once, until an instant-read thermometer inserted in the center of the meat reads 150F, 15 to 20 minutes. Brush with some of the Orange-Ginger Sauce during the last 5 minutes of cooking. Allow the meat to rest on a cutting board tented with aluminum foil for a few minutes before slicing. Slice the meat 1/2 inch thick on the diagonal. Pass the remaining sauce at the table.