I had 2 photographers for my Escape (I paid for the extra one with the photo journalistic package) and I had requested my top three. I ended up with the 2 I really wanted, and I even had one of them for the DP. I may have just been lucky, so I am not sure what the likelihood is
I'd say its probably very likely. I had requested a certain photographer and was told firstly, that they can't guarantee a photographer and secondly, she was already requested by someone else so i wouldn't be able to request them. They then asked who my second choice was.