I have 18" wide shelves and hang two T8 shop lights over each shelf and they work just fine for me. I use one 3000k and one 6500k tube (not LED) in each fixture to try to provide a wider light spectrum. It does mean turning trays of plants around every day as they slightly lean toward the stronger tube but I don't mind as I use the time to check watering and plant health. Been using these for years.
Granted, I place the plants in a line directly under each fixture and don't cover the whole shelf with plants so you could say there's some unused area. But I've never tried doing that so I can't say plants in the middle wouldn't have enough light. There would be room for a third fixture if the fixture had a narrow reflector.