i don't recall the exact specs, but you need a relay because the draw or load of two horns vs. the OEM (one) is technically more than the wiring and switch can handle.
You are in jeopardy of burning up the switch, wire, or both.
Also, with a higher draw on current, you may not be able to supply both horns with enough power to fully enrgize both horns, and as a result, merely be achieving the same dB output as a single horn would achieve.
zero sum game. and worse than that, no benefit, and risk of melted components, or worst case - a fire.

make sense??
gr