Multiprocessor Scheduling is NP-Complete
The deadline is maximum time a single processor can take to complete its tasks (the subset allocated to that processor) where as you are considering deadline as the total of each processor's time.

Let's say there are 6 tasks each with 1 unit of time. Then your example of taking deadline by half of total time of all tasks become 6/2 = 3. Now comes the number of processors which you take to be 2. The task now is to divide the 6 tasks into 2 groups such that the total time of each of the 2 groups is less than or equal to deadline. You cannot compare the sum of total time of each group time with the deadline as the deadline is about the maximum time a single group can take.

In terms of showing that the problem is NP-Complete, I guess you will have to show a reduction of your problem to any of the known NP-Complete problems. Intuitively, I think that this problem can be reduced to a form of subset sum problem which is NP-Complete.