Lena,
You're right. This needs some clarification. First, in general, if you are taking desferal for longer than 12 hours at a time, which I assume is the case when taking it IV, I suggest splitting the dose into two because vitamin C washes out of the body fairly quickly, so to maximize the effect of the vitamin C, take it in two equal dose (not to exceed 250 mg daily) while taking the desferal. In this specific case I am concerned about the possibility of high heart iron, so splitting the dose will minimize the potential for releasing too much iron into the heart at once.
For someone using desferal for a routine period of around 12 hours, the C should be taken in one dose at the beginning of the desferal administration.