Discovered that the RG-59 half of a siamese camera run was bad. We have a second, functional siamese run used for another camera. I'd like to power both cameras with one of the 18/2 halves, and send the second video feed over the other 18/2 run. Any suggestions for a part to do this? The run is about 75 yards through conduit to a Wirepath DVR.

The 700 series pulls a lot of power in the dark. 75 yards is a LONG way to pull the power for 2 over 18/2.
If you are using 12VDC to power them, I would be willing to bet you will have problems at night. Including possibly cameras burning up on you from low voltage over time. You will need to power them with 24VAC, it's still pushing it.

As far as using the 18/2 to send video, it might work. That distance thou is likely again to cause problems. All you can do is test and see if it will work. The likely problem you will run into is interference and a loss of picture resolution.
Try these:
[Link: nvt.com]

Thanks!