The rules do say "a training flight" (singular).
However, if one completes two flights with less than 30 minutes between, JAR-FCL states that the flights should be recorded as a single entry.
Thus a single log book entry of say dep 0900 arr 1030 flight time 1 Hour can be entered for both a single 1 hour flight (with a lot of taxi time) or 2 * 30 min flights with 30 min between. Who can tell other than the person making the entry?
So.....apply common sense, make sure that the time between flights ensure that you can legally record them as a single entry and get the most from the training provided.
I don't think that the JAA or CAA would wish to in any way penalise a pilot who had to call a flight short due to say weather and make that pilot repeat those parts covered in a subsequent full hour - provided that the instructor did not think that the standard displayed would warrant some more time.
Regards,
DFC