If I have two lights with a range of ~250m and I put them next to eachother on full power, do I get more range or just more brightness in the same range that they individually already had?
My guess - Same range, 1/3 more brightness.
Apart from what you describe as range, the brightness (amount of candela) at that range is double. Because now you have two identical sources of the original light. The way you perceive that brightness is another matter. It’s the way the human eye operates. You might perceive it as 30% stronger.
Candela equations are not linear, because at double distance, the light shines at 4 times the surface. You can say that the (ballpark) throw of the double unit is square root of 2 (is 1,41) times the original.
I would say lumens will double, candela will stay the same.
lumens double
candelas will double if you can place the beams on top of each other
range will also double if you can do that
because your target will get twice as much light
or is a “distance-squared” issue?
i think not
Yes it is.
At twice the distance you can light twice as much wall in hight and twice as much wall in width.
So the lighted surface is four times larger than original. (2x2).
While the lighted surface at twice the distance will be four times larger it will also only be illuminated at one quarter of the initial brightness since light intensity has an inverse square relationship to distance.
Double the lumens, double the candela, around 1,5 times the throw.
Henk4U2 posted the ideal theoretical number for the increased throw distance from using two identical lights as being the "square root of 2" or ~1.414 more throw distance.
Using the same inverse square relationship of intensity to distance, it would theoretically take four identical lights to double the throw available from only one light.
But as they say, "In theory there is no difference between theory and practice but In practice there is" and since we are not in an outer space vacuum, our atmospheric conditions like fog and air quality issues like suspended particulate matter will reduce the perceived increase in throw preventing us from clearly seeing much farther with multiple lights.
I carry two and usually three lights on night walks in the woods to have backups. When I use two lights at a time, it is for covering more area by pointing them in different directions and I had not really noticed much more throw when they were pointed in the same direction, but this thread has me reconsidering and I have multiples of my favorite lights that I can experiment with.
Two lights were helpful when I crossed paths with some aggressive raccoons who kept slowly advancing towards me, so I pointed one light at them on strobe confusing them while using the other light aimed in the opposite direction to see where I was retreating to.
Not a stupid question at all. Very good question in fact. Thanks for the enlightenment. 
The FAQ from "HDS Systems" includes useful information especially the answer to the question, "How can I see further?":
The method and explanation are too long to quote in their entirety, but basically you use the lowest brightness level possible and after your eyes are adjusted to that, you will temporarily be able to see better and farther when you increase the brightness level when you really need it, much better and way farther than you would ever be able to see after your eyes adjust to this new higher brightness level.
i agree, it is a very good question, i was wondering about that myself.
We know for sure that the object at 250 meters that is lit up at moonlight level with one light will now be at least”double” moonlight level with two lights. How much farther are you going to have to go to get to one moonlight again? Probably not far. Because you’re going to double the light over the entire distance the usable range is going to increase. The reflectivity of the objects are going to have a larger bearing on how much more distance you can see. I don’t think math is going to give you a good answer. How far can you see now with a light that’s rated for 250 meters? I just did a quick and dirty test outside with two identical lights at roughly 2 lumens and by eye I couldn’t detect a difference in total throw. Trees that were out of range with one light did not become visible with two lights. This should be somewhat easily testable with low levels in fairly short spaces with or without equipment. Although no two lights are going to put out exactly the same lumens.
The best thing would be to actually try it with 2 identical lights, take photos in manual mode with same settings. trying different distances, and different brightness levels.