You can try with:
SELECT MAX(location_a) AS location_a,
MAX(refno_a) AS refno_a,
MAX(location_b) AS location_b,
MAX(refno_b) AS refno_b
FROM (
SELECT location AS location_a,
refno AS refno_a,
NULL AS location_b,
NULL AS refno_b
FROM tableA
WHERE refno = '1234'
UNION ALL
SELECT NULL AS location_a,
NULL AS refno_a,
location AS location_b,
location AS refno_b
FROM tableB
WHERE note = 'LN1234567') s