Design for table with multiple links to other table
Site1 and site2 can both be FKs, linked to the ID_site PK. This works on any rdbms that i'm aware of.

Example Query:

SELECT CT.site1, CT.site2, S1.site_description, S2.site_description
FROM circuit_table AS CT
INNER JOIN site_table as S1
  ON CT.site1 = S1.ID_site
INNER JOIN site_table as S2
  ON CT.site2 = S2.ID_site