If you're asking what are the odds of City drawing United once City had already been picked out of the hat, then the answer is 1/63.
If, on the other hand, you're asking what the odds of them being the first 2 teams out the hat (before any balls have been drawn at all), then the answer is 1/4032 (i.e. 1/(64 x 63)).
HTH (source: A* GCSE back in the day ).