If I understand you correctly, you are trying to free up the tuner that is recording the first program so it can be used to record the second program. The best way to achieve this is to give the first recording lower priority than the second recording. When it is the time to record the second program, MC will force the first recording to stop and start recording the second program.
You can also manually adjust recording time by changing the "keep recording after the program ends" to a negative number (to shorten your recording by one hour, enter -60 minutes).