I have a pair of KX-170Bs and a KA-134 audio panel. When I transmit on #1, it bleeds over and transmits on whatever frequency is selected on com #2. The only way to use #1 to transmit is to shut off #2. I've tried 2 audio panels with no change. I've swapped the #1 and #2 radios between trays with no change. I've narrowed it down to the wiring/antennas, but I'm not sure where.

Any ideas???

A: a bad antenna on #1, check the antenna, coax and connectors. it could be a loose connector, or the coax could be chaffed where it is rubbing somewhere, or the antenna could be bad.

B: whoever wired the radios did something really bizarre when they were installed.

I think you might have described it incorrectly. I am quite sure number two is not transmitting at the same time. If it was, it would be a simple PTT wiring error.
I think what you meant to say was you hear stuff you shouldn't, on Com 2 when xmit on Com 1.
Have the avionics guy check what is called the inhibit wire. It reduces sensitivity on Com 2 or 1 while the other is transmitting. It is a one wire connecting on the harnesses. Easy to ring out....

So did it always do this or did it just start happening? One thing that does happen regularly is corrosion under the antenna. You can pull the radios out of the rack and check the resistance between the center conductor and ground, it should be infinite. If it has some resistance, then I would start at the antennas by pulling off the coax there and see if you still have your resistance. If it goes away, then your antenna is toast. The antennas are basically a wire, cut to a specific length, inserted in a fiberglass rod on a baseplate, then they put the fairing over it and inject foam. The foam can get saturated with water over time, causing all kinds of havoc. The line some radios have is called an "inhibit." Generally it is used on the DME, but some older radios used it for the receive as well. If it is just breaking through, you may be able to adjust the squelch on #2.