Heyicspots! Yes, I would love to have put the 2 horns in there, but the damn wire was so small, that I could not pull it out some to try to split it.. Plus it is such a hard tight spot to work that I was very glad just to get one in there!
Can you send me photos of yours

 

What you need to do is build a splitter/extension that plugs into the stock wiring. Get one male spade connector, two female spade connectors and about 12" of wire. Cut the wire into two ~6" sections and strip off all four ends. Crimp the female spade connectors to the end of each wire. Next Insert the other end of both wires into the male spade connector and crimp it. Once the horns are in you will plug the male spade into the stock wiring while the female spades will go to each of the horns.

For the bracket you can attach the bracket that came with your new horn to the same bolt that attaches the first horn to the stock bracket. It's much easier to remove the stock bracket from the car before doing this. also you can connect the ground wires to the brackets where either horn is bolted on. The end result will look like this.



This picture is from droyal at mazda3forums. He added the Fiamm FB low tone to the stock high tone. It might take you a few times putting the whole apparatus in the car to get the angles of the brackets correct so that the second horn doesn't hit anything, but eventually you'll get it. Let me know if you need any more pictures or instructions.