Count instances of value in multiple tables
One way would be to pivot  

WITH CTE AS
(
SELECT [Document No_], 'SL' AS Source
FROM [Sales Line]
UNION ALL
SELECT  [Document No_], 'SLA' AS Source
FROM [Sales Line Archive]
) 
SELECT P.[Document No_],
       P.SL,
       P.SLA
FROM CTE
PIVOT (COUNT(Source) FOR Source IN ([SL], [SLA])) P
WHERE P.[Document No_] IN (1,2,3);


I'd hope that the predicate on [Document No_] gets pushed up into the selects from the source tables. Check the execution plans to be sure.