The only considerations should be about the light coverage at the bottom of the tank. Is it high enough that the light covers the bottom and doesn't 'spill' or waste a lot of light going outside the tank. And is it high enough off the water so it doesn't get coated with salt creep.
Back years ago, before leds had dimmers, the height off the water was the only way to make the light more or less intense in the tank. Now we have control over that so height is a much simpler issue.
If you can't get good coverage over the length of your tank without 'spilling' a lot of light out the front and/or back, then you may want to go with 2 fixtures.