Replacing one ramp door cable

Did a search here and on Google and could not find the info I wanted.

I have a used, 2004 Haulmark 8x20 Race Trailer I am re-habbing and it only has one ramp door cable. It works fine - tension, etc. - but I would like to put the other cable on for safety, maybe even both since Haulmark sells them as a pair.

My search kept turning up stuff on adjusting the cable by way of adjusting the spring tension. Not necessary I don't think, IF there is only one spring or if the tow are connected in a way that tension is created on the left spring when the door comes down. Right side is fine. I just need to install a cable on the left side.

Because the spring tension is already good, I am hoping I can just thread the new cable into the spring drum, let the door come almost all the way up, and attach the eye end to the door.

Plausible? Tension imbalance? Are springs separate? (entire spring/drum/shaft assy. is covered by interior aluminum)

If so, it seems I could put the new cable on the left side, go up and down with the door a couple of times, and then use the same process to replace the right side cable, though it is possibly under more tension.

DK

Replacing one ramp door cable