Это не будет быстро:
SELECT x.UserID, x.UserName, y.LogTime, x.LogTime, y.LogDate
FROM (
SELECT Test.UserID, Test.UserName,
Test.LogTime, Test.LogDate,
(SELECT Count(*) FROM Test t WHERE t.LogTime<=Test.LogTime) AS c
FROM Test) AS x
INNER JOIN (
SELECT Test.UserID, Test.UserName,
Test.LogTime, Test.LogDate,
(SELECT Count(*) FROM Test t WHERE t.LogTime<=Test.LogTime) AS c,
c+1 as k
FROM Test) AS y
ON x.c = y.k
WHERE x.c Mod 2=0