うーん、それが最善の方法かどうかはわかりませんが、既存のクエリに簡単に追加できます:
SELECT *
FROM products
WHERE p_show='Y'
AND p_id IN (SELECT p_id
FROM prods_2cats AS PC
WHERE PC.cat_id = $cat1_id)
AND p_id IN (SELECT p_id
FROM prods_2cats AS PC2
WHERE PC2.cat_id = $cat2_id)