Problem with transactions

I want two users to execute the following queries:

1) delete from forum where id = 'A';
1) insert into forum (id, name) values ('A', 'testa');

2) delete from forum where id = 'A';
2) insert into forum (id, name) values ('A', 'testb');

id is a unique key. The numbers designate the user (1 and 2). The problem is
that I don't know in which order the queries are executed (I only know each
user executes its DELETE query before the INSERT query). I can't use UPDATE
because I don't know in advance that there exist a row with id A.

How do I prevent the queries from being executed in the wrong order and thus
causing an 'duplicate key violates unique constraint' error?