Yes. Technically In your example, technically speaking, you are really performing another round of rolling - rolling zero dice - and then resolving the final task. The die you have secured in your spell is a usable part of your dice pool, thus allowing you to meet the requirements of the adventure task even though you didn't physically roll any dice. Remember that the spell is discarded after you use the reserved die in that way.
However, if you still have more unfocused/unreserved dice to roll, you do have to roll them. In your example, you had exactly no dice left to roll after completing the first task and when moving on to the second. If you did have dice left, however, you'd have to roll them, which gives the risk of rolling a Terror result, which could have consequences. Thus it's important to note that, strictly speaking, you cannot complete multiple tasks at once (and in your example, you are not really doing so).