Dual Primary keys

Hello can any body tell me how to code a table with dual primary keys? The table 'borrower' has two PKs each one has been used in other tables within the same schema as single PKs

Here is what I have tried:
create table borrower(user_id int primary key,
issn varchar(15) primary key);
Error: create table / primary key multiple primary keys for table 'borrower' are not allowed.

I also tried this:
create table borrower(user_id int ,
issn varchar(15),
primary key user_id, issn);
ERROR: parser: parse error at or near "user_id" at character 68
and
create table borrower(user_id int ,
issn varchar(15),
(primary key user_id, issn));
ERROR: parser: parse error at or near "( " at character 56
thanks
Janet