Short answer... yes you can but no you shouldn't.
Long answer... this has been argued over several times, basic thought it that although it will work fine to just splice in the extra horn you're near the limit and you should have a relay and fuse if you're going to add a second/bigger horn. Car manufacturers use relays for a reason, and they're pretty basic to fit. I'm sure if you use the search function you'll find several threads covering the how-to of putting a relay in.
