I have a 4" computer fan on my lights and for flow, as long as they don't blow against each other, the heat will vent out, meaning 1 goes in and 1 goes out. You might find that only 1 fan is enough as long as there is only one opposite opening, air will move though the whole fixture . As for which direction to aim, it doesn't matter for cooling. Though I find that salty air will be drawn out, so it can rust out your fan on the exhaust side, so in my setup, the fan draws in fresh dry air from outside the tank, and blows it in, even though the now hot air is blowing onto the water surface.
This actually causes a better evaporative cooling then when I had the air blowing out, but like I said earlier, the salt was corroding the fan, so I had to flip it around.
PS, when you install your fans, it'll be more effective if you can cut out a full size vent instead of using the holes.