To make the swordfish marinade, juice one of the limes and slice the other into wedges. Set wedges aside. Whisk to combine the lime juice, olive oil, apple cider vinegar, green onions, and salt. Set swordfish steaks in a shallow dish, pour marinade over swordfish, cover, and refrigerate. Marinate steaks 15 minutes, flipping in marinade halfway through. Marinating longer is not advised, as the acids in the marinade will start to cook the exterior of the fish (think ceviche).