I can't speak for my own experiences, but I know that this combo is a bit unusual... so I went to read some published studies and I got my answer. They have been published at the beginning of last year but are based on research made by another team around 2005 so it is not new new.
If I may make complicate things simple, they notice that some AZA/6MP users produce more of a certain metabolite(6MMPN) than others. That metabolite is known to be hepatotoxic (affects your liver). Introducing the allopurinol apparently helps getting a better ratio between 6MMPN and 6TGN(another metabolite that helps the Crohn) and may therefore be responsible for an increase of the therapeutic effect. The studies state this " they showed a general improvement in clinical outcomes (reduction in steroid usage and improvement in disease activity scores) and a normalization of liver function tests."(Sparrow(2007) in Gardiner, Geary, Burt and al.(2011))
Did they reduce your AZA dose? Cause it specify that everywhere, AZA needs to be reduced by 25-50% of the dose they would normally give you because the Allopurinol will makes the AZA level in your blood higher than they would normally be so be careful about that. On the same idea, be really assiduous with your blood test. This combo may mess with your WBC big time so you don't want to neglect any blood tests!!