Deb,

Connect the negative side of your flasher directly to the ground connection on each unit.

On the positive terminal of each unit, attach a diode.... connect the other end of the two diodes together and connect the postive lead of your emitter to that junction.

If you hook the diodes up backward, the only problem you will have is that it will not work.... just reverse them in that case.

The purpose for the diodes and to allow current flow from either unit through the emitter through its diode while the other unit effectively blocks a path back into the non-sending device.

No big deal to do. The diodes will lower the voltage to the emitter a little but, but I have not had a problem with that in an application.