I think that you will find that your car has a single tone horn, ie a Hi tone, maybe what you want/need to do is to fit the complimentary Lo tone horn, now you should know is that these horns are driven from a controller, and doubling the current draw from controller output might blow that output. So, try to find out which way to achieve what you want before messing anything up, I think that using the power feed to the first or original horn to control a relay for the second horn will work okay as long as you check that there is a quenching diode fitted across the coil of that relay, take a new fused feed from a convenient point to drive the second horn.