Usa row_number() función y hacer el condicional agregación :
select id, IdCust, Ref,
max(case when Seq = 1 then stock end) as [Stock A], -- second table *id*
max(case when Seq = 1 then code end) as [Code 1],
max(case when Seq = 1 then price end) as [Price1],
max(case when Seq = 2 then stock end) as [Stock B], -- second table *id*
max(case when Seq = 2 then code end) as [Code 2],
max(case when Seq = 2 then price end) as [Price2]
from (select f.*, s.Id Stock, s.Code, s.Price,
row_number() over (partition by f.Ref order by s.id) as Seq
from first f
inner join second s on s.Ref = f.Ref
) t
group by id, IdCust, Ref;
Sin embargo, este iría con conocido valores de lo contrario necesitaría ir con dynamic solución para eso.
