O.K., here is the deal. Both of my horns work. BUT not together. I have to unplug the connector to one of them to make the other horn blow. If both horns are connected at the SAME time then they to not work. It seem like there isn't enoughvoltage and/or current to make both of them to work together. What do I need to check? Is the a horn relay on the 1966 models? Better grounds?
Please help.

sounds almost like theres a short in the wiring.

you should just use the one that sounds the funniest. i remember playing around with mine when i was installing them and if i remember one of them is pretty funny sounding.

hmm .. maybe check the following
disconnect both horns. run a temporary cable to both horns (splieced together) and connect this to battery+
see if you get both to sound at the same time. that might help isolating the problem

you might have weak horn spring contacts or worn steering wheel contact rings. Also try removing the horns and cleaning up the mating areas from the horn brackets to the mounting area on radiator support to have good ground contact.My steering wheel was cracked and worn . I pulled it off of the column and found that the 2 metal contact rings were very worn and pitted. I am buying a repro wheel and hope it takes care of it. I also bought a turn signal switch to replace the horn spring contacts.

They may need to be adjusted. The horns are designed so that they need a very particular voltage range flowing through them in order to function, and the back of each horn has a small screw that you can screw in and out to adjust them. Technically the manual says to check for a certain voltage with the horns on and adjust until it's in the proper voltage range, but the easier way is to back them all the way off, make sure it's clean, and then just barely set each one with minimal tension. Then with them both plugged in have someone sit on the horn as you adjust each screw in a bit at a time until the horn starts to work. Since each one works but only when 1 is plugged in and not both, I'm willing to bet it's horn adjustment.