Another idea struck me...
(It only hurt for a couple minutes...)
Requires three people, two rifles with 'decent' scopes, and two arrangements to 'sand bag' rifles in place. Walkie-talkies would be helpful. Make sure all the guns are empty and so forth; be sure everyone involved are 'friends'.
Sand bag rifle number one in place, pointing down range, at extreme right edge of proposed firing line. Very important, this rifle must be at 90 degree angle to firing line.
Sand bag rifle number two in place at extreme left end of firing line. Make sure the butt of rifle number two is on the same line as the butt of rifle on that 90 degree angle to the firing line.
Surveying guys do this with transits, which are very similar to telescopic rifle sights, but without rifles. My plan here is done cheap, assuming you have access to two rifles with 'scopes.
Person number three is a very trusting sort. He is holding a marker stick down range. His stick has to be directly downrange from rifle number one.
If you've done this right, you've created a "right triangle", defined by the two rifles and the brave lad with the marker stick.
Carefully measure the distance between the two rifles. You know rifle number marks a 90 degree angle. You have to measure the angle of rifle two to the firing line. Knowing two angles and the length of one side of a triangle will allow you to calculate the length of the third angle (which really doesn't matter to us in this case) and the lengths of the other two sides of the triangle, which is the point of this whole thing.
The distance between rifle number one and rifle number two (the firing line width) is the "adjacent" side of the triangle. The distance from rifle number one to the marking stick is the "opposite" side; this is the range from firing line to target. The distance from rifle number two to the stick is the hypotenuse.
Don't you wish you paid more attention in High School?
What you really want to do is to preset the angles of both rifles and the firing line length. Then the intersection of where the two scopes are pointing will be what ever range you want.
We'll say you have a range 50 feet wide and you want a 100 yard target distance, okay? So you have a triangle with a 50 foot adjacent side hooked to a 300 foot (100 yards) opposite side by a 90 degree angle, right?
What you really need to know is what angle to set rifle number two in order for both rifles to be pointing at the same place at 100 yards.
You can do this a couple ways.
One. Draw this out on paper to scale. Draw a right triangle with the short side equal to 50, the middle side equal to 300 and connected by a 90 degree angle. Measure the angle of the short side to the hypotenuse (the longest side) and write that down.
Two. Calculate it.
The tangent of the angle in question is equal to dividing the opposite by the adjacent, or 300 divided by 50; or 6. Checking a 'trig table' (do a websearch) gives an angle of 80.5 degrees. (This is only if the firing line is 50 feet wide, remember.) Setting up rifle two at exactly 80.5 degrees is going to be tedious, too.
Set up your two rifles and have your brave stickman walk out and then you two guys with the scopes wave him back and forth until the stick is lined up in the zero of both rifles. (This is where the walkie-talkies would be handy.)
This may be more math work than you want to do. But it will not fail as long as you measure and calculate properly.
Maybe you can find a surveyor who is a shooter and trade him some range time?