Triggers?

I have two tables

alternativa1( id_altern int4 primary key, ....)
alternativa2( id_altern int4 primary key, ....)

I want a trigger that doesn't allow insertion of a value in one table if it
exists on the other, i.e., the following operation must be prevented:

insert into alternativa1 values(1,...)
insert into alternativa2 values(1,...)

Any suggestions on how i can implement this feature?

Paulo