Isso funcionará para o seu caso, eu acho:
SELECT
t1.PositionDate,
t1.Currency,
COALESCE(t1.Value,t2.value) AS Value
FROM t1
LEFT join (SELECT MAX(PositionDate) AS PositionDate,Currency FROM t2 WHERE PositionDate < t1.PositionDate GROUP BY Currency) tjoin
LEFT join t2 on tjoin.currency = t2.Currency and tjoin.PositionDate = t2.PositionDate
