Unlike some other frameworks, RADICORE does not expect primary keys to be limited to a single column, so it should not have any difficulty in handling compound primary keys as well as compound foreign keys.
The SQL query that the LINK1 pattern tries to build is shown at http://www.tonymarston.net/php-mysql/many-to-many.html#a5b such as
SELECT a.a_id, b.b_id, b.b_desc,
CASE WHEN x.b_id IS NULL THEN 'F' ELSE 'T' END AS selected
FROM a
CROSS JOIN b
LEFT JOIN x ON (x.a_id = a.a_id AND x.b_id = b.b_id)
WHERE (a.a_id = 'whatever')
What you need to do first is to build an equivalent query for your table structure, then try it out in your database client (phpMyadmin or HeidiSQL or whatever). When you get this working you should then see what query RADICORE builds by default. You will only need to take corrective action if the query is not correct. You certainly should not have to use CONCAT to construct a dummy primary key containing a single field.