Without any load at all there won't be any voltage drop measured at the far end. As you start piling on devices, and adding to the current flow, the voltage will start to drop in a predictable ratio (Ohms Law).

You can increase the voltage at the power supply but you have to be careful. If one thing is drawing current all the time and the others are only momentary then you could eventually burn out the full-time device by running it at a higher voltage than it's rated for.

As far as doubling up conductors, here is good rule of thumb:
When you double up two conductors of equal size the resistance is cut in half.

You can search online for the resistance of a single conductor of Category cable. I think it's around 0.026 Ohms per foot. As long as you know how Ohms law works this should be fairly simple.

Many power supplies for CCTV cameras and low-voltage lighting have multiple secondary taps just for this reason. But you can't just jack up the voltage as high as you want without calculating the parallel conductor resistance, the continuous current draw, the momentary current draw, the low and high voltage range maximums of the devices, the length of the cable run, and the ambient temperature of the cables in the raceway.

Aren't you glad you asked?

ERNIE, As always, please correct any misinformation or missing information that might be contained in this answer.