I installed dual "Freeway Blasters" on my Impala back in 03, it only had one horn originally. I have had to lay on it for up to 8 or 10 seconds trying to get the attention of an inconsiderate driver before. Have not replaced the fuse or the relay for the horn as of yet. Also I pointed both horns through the grill, the original pointed at the radiator core support.
Update: I ordered up a pair of HELLA 007424801 Twin Trumpet High/Low Tone horns from Amazon, $24. Made up a new wiring harness/adapter from the connector attached to the original GM horn, to connect between the stock GM 2-terminal female horn connector, and the male spade lugs on the Hellas, so I didn't have to modify the Imapala's wiring at all.
Hella insists that you use the mounting bracket that is attached to each horn from the factory, for electromechanical and warranty reasons, which made things a little tighter space-wise, so i used the original GM mounting bracket PLUS the Hella brackets, which kinda hung the horns way out there at bit, but all in all it wasn't a bad result. (Would've been very easy to take the brackets off and mount exactly as the originals, using the little bolt right on the body of the horn itself, without the brackets). The horns are great, kinda muscular but not too loud.