Heat 2 very large grill pans or griddles to high heat.
Mix together the salt, oregano, chili powder, black pepper, cumin, garlic powder and cayenne pepper in a bowl.
Put the steak cubes and shrimp in separate large bowls. Sprinkle the spice mixture evenly over both and toss to ensure the steak and shrimp are well coated.
Add some oil to the grill pans and, working in batches, grill the shrimp on one grill pan until they turn from translucent to opaque, and the steak on the other grill pan to medium rare, 3 to 5 minutes per batch. Clean and re-oil the grill pans between batches.
Serve with the taco bar fixings of your choice.
This is a large-quantity recipe for feeding a crowd, but it can be scaled down quite easily.