Here's one way to get the specified result:
SELECT s.username
, s.acctsessionid
FROM (
SELECT r.username
FROM radacct r
WHERE r.acctstoptime IS NULL
GROUP BY r.username
HAVING COUNT(1) > 1
) d
JOIN radacct s
ON s.username = d.username
WHERE s.acctstoptime IS NULL
ORDER
BY s.username
, s.acctsessionid