There's a particular spot at my home QTH that would be good for a
transmit antenna for 2 meters / 70 cm. Right now I'm picturing a j-
pole made of copper tubing in that spot. The only hitch is that one 20
to 25 ft long section of the coax feedline to this antenna will need
to run parallel to the coax feed to my hf receive-only loop. Within
this 20-25 ft long span, the feedlines would be separated by about a
foot.

How concerned should I be about this, given that I'll probably never
be using be using these antennas at the same time. I'm guessing the
effects would be negligible. Am I wrong?