As i said a single relay wont do this without a controlling circuit .....as you have discribed a controlling circuit.
Really we are wasting our time unless the OP can bother to explain what it is he wants.
In most countrys it is not legal to have any other switch on a horn other than the original single switch, as it is deemed as a emergency warning device.
As long as the horn defaulted to a tone of one or the other with a single push of the original horn switch then it should comply with regualtions.
But if you needed to activate a second switch to operate the horn than no it would not comply with regulations.
There used to be a siren driver chip around some years back that created a He-Hor-He-Hor, to a horn or other emitters without the need for dual horns.
There is also the air horn kits that do this as well with trumpet lengths and air flow.
Still no reason a simple 555 delay circuit wont work with dual horns.
Pete.