I think taking both increases risk of some cancers . . . by a slight degree. That's pretty much all I know about taking 6-mp with humira.
I know plenty of people with rheumatoid arthritis take mtx + remicade/humira/enbrel/cimzia/orencia . . . in fact, I think with RA, mtx is pretty much a given, and if that doesn't help, they add on a biologic. So, it certainly isn't unheard of to take a biologic with another immune suppressant (such as mtx, or 6-mp or imuran). I think there are other people on the crohn's board who are also taking a biologic + an immune suppressant (6-mp/imuran/mtx).
My husband who is currently struggling with possible IBD related arthritis, is likely looking at adding mtx to control the arthritis, in addition to the remicade that he is already on. So, it isn't unheard of, to mix 2 such meds together. Yes, it does increase the risk of certain side effects. . . but when you have crohn's, you live in the little spot that is between a rock and a hard place . . . so I'm sure you understand that sometimes we just have to take our chances.
Husband with Crohn's
Diagnosed March 2003 Ulcerative Proctitis
Diagnosed March 2008 Crohn's & C-diff, hospitalized 45 days
Canasa, Lialda, Remicade, VSL#3, Florastor
In Remission since June 2008
Stopped vancomycin for c-diff Jan 1 2009
C-diff free, until Sep 2, 2009
Fighting c-diff, I guess for life