Load weight on a horizontal wireframe ladder

I don't have any formulas for you, but I can mostly answer your question. If both the 15 meter and 2 7 meter ladders were both the same distance they would experience the same strain, weight and load as each other.(unless the connection between the 2 7 meter ladders was weaker than that of the middle of the 15 meter ladder) A person would cause the most strain on the ladders when they are in the middle of them, and the least the farther away from the center they are. I don't know how much load the ladders can take A, because I don't know how strong the ladders are and the supports keeping them up, and B, I don't even have an equation to calculate that. I hope this answers you general question even if I can't give you any formulas.

The problem is complex enough to need computer simulation for a reasonable answer.

As a first stab at it you can look at the wire ladder (black lines) spanning a distance $S$ between points A and B below:

The middle support (blue line) connects to the ladder at C and to the ceiling at D. The load $W$ is at point G a distance $x$ from A.

If the angle $varphi$ is near zero, then the support forces $F_A$ and $F_B$ need to be really large to counteract the weight $W$. The angle increases as the ladder and/or the supports stretch. The higher the load the more stretch

The solution will be found when the elastic forces match the forces from the sketch above