I'm on both. I actually started the Remicade first because I needed to get out of a severe flare - FAST! Then I started the Imuran. It's pretty common practice to be on both from what I understand because the azathioprine helps to reduce the risk of building up antibodies to the mouse proteins in the Remicade - thus rendering the Remicade useless. I've been on the Remicade for 2+ years and the azathioprine slightly less than that. I've been doing great! My doc is hoping to wean me off the Remicade completely (not before Humira is approved in case Remicade won't work 2nd time around) and leave me on the Azathioprine. He likes it better this way because he feels there is more long term data on the aza than on the Remicade.
Good luck! Hope it works as well for you as it does for me! :)