MySQL: Optimize UNION with "ORDER BY" in inner queries
Just out of curiosity, can you try this version? It may trick the optimizer to use the same indices that the subqueries would use separately:

SELECT *
FROM
(SELECT l.id, l.account_id, l.vnum, l.count, l.preis, l.zeitpunkt AS zeit,
 'hp' AS source FROM is_log AS l WHERE l.account_id = 730
 ORDER BY l.zeitpunkt DESC LIMIT 10) 
    AS a

UNION ALL

SELECT *
FROM
(SELECT l.id, l.account_id, l.vnum, l.count, l.preis, l.zeitpunkt,
 'ig' AS source FROM ig_is_log AS l WHERE l.account_id = 730
 ORDER BY l.zeitpunkt DESC LIMIT 10)
    AS b

ORDER BY zeit DESC LIMIT 10;




I still think that the best index you could have is the compound (account_id, zeitpunkt). It would yield the 10 rows fast, and no tricks would ne needed.