Portion the meat into two servings, one for the grill, and one for the fajitas. Marinate the fajita portion.
If you want both grilled, and fajita flavors in the fajitas, grill the steak, cot the fajita slices and set aside. Cook to medium rare at most. Make a flavorful au jus with fajita seasonings added. Heat the au jus to simmering. Add the beef slices just long enought to heat the meat, about a minute or so. Cook the onions, peppers, and whatever else you will be puting in your fajitas befre heating the meat in the au jus.
Marinades have acidic ingredients in them that will toughen the meat. The au jus is basically a beef broth brine that will flavor it without making the meat tough.
Seeeeya; Chief Longwind of the North