the short anseris that you cant. Splitting a task does not create any new attributes- it is still one task, and can only be formatted a such.

As an alternative (and for me a better one on many levels) is to copy the task and inset it under the original, so that you have an exact copy of the original task. Link these together and set durations to be the same as those of the first and second halves of the original task.
this will enable individual formatting, but more importantly maintains the programme. Split tasks never work well in my experience, as the second half tends to be dependent on other tasks being completed (as a real life situation, or it wouldnt be split in the first place.